Electrolyte containing electrode interface modification additive and lithium ion battery

Abstract

The invention discloses an electrolyte containing an electrode interface modification additive and a lithium ion battery, and relates to the technical field of lithium ion batteries. The electrolyte is characterized in that conducting salt is lithium bis (trifluoromethylsulfonyl) imide or / and lithium bis (fluorosulfonyl) imide; the additive is lanthanum nitrate, and the mass fraction of the lanthanum nitrate in the electrolyte is 0.05%-0.30%; the solvent is an ether-ester mixed organic solvent. The lithium ion battery runs under the cut-off voltage of 4.5 V and comprises a positive electrode material, a negative electrode material and the electrolyte. The electrolyte system is good in compatibility, the lithium ion battery supported by the electrolyte system can achieve good cycle performance and good safety, and the coulombic efficiency of the first circle of 0.2 C can reach 91.9%.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to an electrolyte containing an electrode interface modification additive and a lithium ion battery. Background Art

[0002] As a hot application in portable electronic devices, power batteries, and large-scale energy storage, lithium-ion batteries have the characteristics of high energy density, long cycle life, low self-discharge, low carbon and environmental protection, and no memory effect, and have broad development prospects. In order to break through the technical barriers to the development of lithium-ion batteries, such as endurance and safety assurance, the development of the next generation of lithium-ion batteries with high safety, high energy density, and longer life is a major issue that needs to be solved in the field of secondary battery technology.

[0003] Higher energy density means that the battery charging cut-off voltage needs to be increased, thereby obtaining a higher battery specific capacity. 0.6 Co 0.2 Mn 0.2 For example, O2 has a charge cut-off voltage of up to 4.5V. The widely used electrolyte is a carbonate-based electrolyte with lithium hexafluorophosphate as the conductive salt. It has strong water sensitivity, a narrow temperature range, and the conductive salt is easy to decompose (decomposition potential 4.2V vs Li / Li + , the decomposition products will catalyze the carbonate ring opening), which can easily lead to an increase in side reactions at the positive and negative electrode interfaces, lithium salt consumption, and reduced interface conductivity, accelerating the generation of negative electrode lithium dendrites and the dissolution and structural collapse of positive electrode materials, further causing problems such as increased internal resistance of the battery, battery heating, and a sudden drop in capacity, and even major safety accidents such as the internal diaphragm of the battery being pierced by dendrites and the battery short-circuiting and catching fire. On the other hand, an excessively high charging cut-off voltage can lead to dissolution and corrosion of the positive electrode aluminum current collector, especially in high-voltage-resistant lithium bis(trifluoromethylsulfonyl)imide (LiTFSI)-based electrolytes. Therefore, it is crucial to develop battery technology that has both high-voltage stability and uniform charge transfer during the electrochemical process at the positive and negative electrode interfaces.

[0004] As the "circulating blood" of lithium-ion batteries, electrolytes are usually composed of conductive salts, solvents and functional additives, and play a vital role in solid-liquid interface and bulk mass transfer. In order to optimize the electrochemical process of the electrode, starting with the electrolyte is one of the strategies with the most research value and space. Among them, selecting appropriate electrolyte additives is the main way to regulate the electrolyte. Specific additives can achieve the effect of adjusting the solvent-solute coordination structure, Li +Direct regulation of solvation / desolvation processes, interphases such as interphase solid electrolyte membranes, etc. at the solid-liquid interface. Conventional organic additives usually only have a single effect. For example, film-forming additives mostly form organic fragments; or they cannot achieve the coordinated regulation of the anode and cathode interface films, that is, they can only unilaterally obtain a more stable window of oxidation or reduction potential compared to carbonates, and there is no absolute advantage for both poles. Such additives will still lead to low Coulomb efficiency in the first cycle of the battery and uneven solid electrolyte membranes, thereby triggering problems such as uneven Li + ion flow transmission and uneven power line distribution at the interface. The battery in this state will experience uneven lithium deposition on the anode and the generation of lithium dendrites during long-term cycling. The piercing of the separator by the dendrites will cause internal positive-negative short circuit of the battery, instantaneous heat release, and thermal runaway. At high charging cut-off voltage, the positive electrode active material is deeply delithiated, the positive electrode current collector is dissolved and corroded, and the electrode material undergoes diffusion-induced stress corrosion. Eventually, the mechanical stress inside the active material and the decrease in the effective contact area of the binder caused by the corrosion of the current collector jointly promote the pulverization and cracking of the electrode and its failure. At the same time, the microcracks generated on the electrode increase its contact area with the electrolyte, increasing the adverse reactions at the solid-liquid interface and electrolyte consumption and pollution, thereby generating a huge interfacial charge transfer resistance. In order to achieve uniform transmission of Li + ions in lithium-ion batteries under high cut-off voltage and uniform distribution of internal stress in electrode materials, there is an urgent need for new electrode interface modification additives applicable to organic systems in order to achieve the stable and safe operation of lithium-ion batteries.

[0005] The selection of electrolyte additives is not limited to molecular compounds, and ionic compounds can also participate in interface regulation. The KongLong research group studied the battery performance with LiNO3 as an electrolyte additive, and the ternary nickel-cobalt-manganese NCM811 battery showed a high-voltage performance of 4.4V. The Ali Coskun research group used LiNO3 as an additive to prepare a lithium-sulfur battery, but the capacity retention rate was only 70.7% after 50 cycles at 0.1C. The Suo Liumin research group used LiAlO2, Li2CO3, Li2SiO3, and Li3PO4 as electrolyte additives to prevent the corrosion of the Al current collector, but it was only used for the exploration of the lifespan of aqueous batteries, and these types of ionic compounds are insoluble in organic solvents.

[0006] Based on the above content, it is necessary to conduct in-depth research on the application of ionic compounds as electrolyte additives for lithium-ion batteries and consider them as electrolyte additives for modifying the electrode interface in high-voltage systems. Summary of the Invention

[0007] In view of the deficiencies of the prior art, the present invention provides an electrolyte containing an electrode interface modification additive and a lithium-ion battery. Lanthanum nitrate (La(NO3)3) is used as the electrode interface modification additive for the stable operation of the lithium-ion battery at a cut-off voltage of 4.5 V.

[0008] To this end, in the first aspect of the present invention, an electrolyte containing an electrode interface modification additive is provided, including: The conductive salt is lithium bis(trifluoromethanesulfonyl)imide or / and lithium bis(fluorosulfonyl)imide; The additive is lanthanum nitrate, and the mass fraction of the lanthanum nitrate in the electrolyte is 0.05% - 0.30%; The solvent is an ether-ester mixed organic solvent.

[0009] Further, the mass fraction of the lanthanum nitrate in the electrolyte is 0.08% - 0.20%.

[0010] Further, the mass fraction of the lanthanum nitrate in the electrolyte is 0.10% - 0.15%.

[0011] Further, the ether-ester mixed organic solvent is made by mixing ethylene glycol dimethyl ether, fluorinated ethylene carbonate and ethyl methyl carbonate in a volume ratio of (0.9 - 1.1):(0.9 - 1.1):(2 - 4).

[0012] Further, the ether-ester mixed organic solvent is made by mixing ethylene glycol dimethyl ether, fluorinated ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1:1:3.

[0013] Further, the concentration of the lithium bis(trifluoromethanesulfonyl)imide or / and lithium bis(fluorosulfonyl)imide in the electrolyte is 0.1 mol / L - 2 mol / L.

[0014] Further, the concentration of the lithium bis(trifluoromethanesulfonyl)imide or / and lithium bis(fluorosulfonyl)imide in the electrolyte is 0.9 mol / L - 1.1 mol / L.

[0015] Further, the electrolyte is used for a lithium-ion battery with a cut-off voltage of 4.5 V.

[0016] Further, the preparation method of the electrolyte includes: Fully dehydrate lanthanum nitrate hexahydrate; Transfer the dried anhydrous lanthanum nitrate into an inert atmosphere glove box for sufficient grinding; In the inert atmosphere glove box, mix ethylene glycol dimethyl ether, fluorinated ethylene carbonate and ethyl methyl carbonate to prepare the ether-ester mixed organic solvent; Add the lithium bis(trifluoromethanesulfonyl)imide and / or lithium bis(fluorosulfonyl)imide into the ether-ester mixed solvent to prepare a mixed organic electrolyte solution. Stir at room temperature. During the stirring process, slowly add the ground anhydrous lanthanum nitrate into the mixed organic electrolyte solution. After sufficient stirring, let it stand to obtain the electrolyte solution containing the electrode interface modification additive.

[0017] In the second aspect of the present invention, a lithium-ion battery is provided. The lithium-ion battery operates at a cut-off voltage of 4.5V and includes a positive electrode material, a negative electrode material, and the above-mentioned electrolyte solution. Among them, the positive electrode material is selected from any one of nickel-cobalt-manganese ternary positive electrodes or lithium nickel manganate; the negative electrode material is selected from any one of graphite or lithium metal.

[0018] Compared with the prior art, the present invention has at least the following beneficial effects: The electrolyte solution containing the electrode interface modification additive provided by the present invention has a regulating effect on both the positive and negative electrode interfaces of the lithium-ion battery. The ether is used to improve the solubility of the additive, and the ester is used to improve the interfacial film-forming ability and provide ideal low-temperature performance. Through the synergistic effect of the ionic compound additive La(NO3)3 and the ether-ester mixed electrolyte solution, a stable interfacial structure is constructed by the additive, and the uniform deposition of lithium on the negative electrode is induced synchronously. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments recorded in the embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings.

[0020] Figure 1 For the substances that may exist after the dissociation of La(NO3)3 in the ether-ester mixed solvent at 0°C - 50°C provided in the embodiments of the present invention (corresponding to La / La 3+ ), relative to Li / Li + equilibrium potential; Figure 2 For the cyclic voltammogram curves of the lithium-ion battery electrolyte solutions in Example 1, Comparative Example 1, and Comparative Example 5 of the present invention relative to the active electrode Al; Figure 3 For the potentiostatic time-current curves of the positive electrode current collectors of the lithium-ion batteries in Example 1, Comparative Example 1, and Comparative Example 5 of the present invention; Figure 4 For the impedance Nyquist diagrams of the positive electrode current collectors of the lithium-ion batteries in Example 1 and Comparative Example 1 of the present invention; Figure 5Schematic diagram of the morphology of the positive current collector of the lithium-ion battery in Example 1, Example 4 and Comparative Example 1 of the present invention; Figure 6 Graph showing the change of overpotential with time of the lithium symmetric battery in Example 4, Comparative Example 3 and Comparative Example 4 of the present invention; Figure 7 The first cycle of the NCM622||Li battery in Example 1 of the present invention at 0.2C (a), and schematic diagrams of the macroscopic morphology of the lithium negative electrode in Comparative Example 1 and Example 1 (b). Detailed implementation manners

[0021] To better understand the above technical solutions, the technical solutions of the embodiments of the present application will be described in detail below through the accompanying drawings and specific embodiments. It should be understood that the specific features in the embodiments of the present application and the embodiments are detailed descriptions of the technical solutions of the embodiments of the present application, rather than limitations on the technical solutions of the present application. Without conflict, the technical features in the embodiments of the present application and the embodiments can be combined with each other.

[0022] In the first aspect of the embodiments of the present invention, an electrolyte containing an electrode interface modification additive is provided, including: The conductive salt is lithium bis(trifluoromethanesulfonyl)imide or / and lithium bis(fluorosulfonyl)imide; The additive is lanthanum nitrate, and the mass fraction of lanthanum nitrate in the electrolyte is 0.05%-0.30%; The solvent is an ether-ester mixed organic solvent.

[0023] For the electrolyte containing an electrode interface modification additive provided by the embodiments of the present invention, the additive La(NO3)3 has a regulating effect on both the positive and negative electrode interfaces of the lithium-ion battery: assisting the positive current collector to generate Al(NO3)3 precipitation, effectively preventing the erosion and dissolution of the Al matrix by the conductive salt LiTFSI, effectively blocking the electrochemical and chemical corrosion paths, and reducing the Coulombic efficiency loss caused by the corrosion of the current collector; assisting the formation of the negative electrode interface film, cleaning the surface of the copper current collector, and avoiding the loss of active Li + due to the formation of the solid electrolyte film during the previous activation process, and reducing the overpotential of lithium deposition on the negative electrode.

[0024] For the electrolyte of the embodiments of the present invention, ether is used to improve the solubility of the additive, and ester is used to improve the interface film-forming ability and provide ideal low-temperature performance. Through the synergistic effect of the ionic compound additive La(NO3)3 and the ether-ester mixed electrolyte, a stable interface structure is constructed by the additive, and at the same time, the uniform deposition of lithium on the negative electrode is induced. The positive and negative electrode interfaces maintain good ionic conductivity, and the internal liquid phase of the electrolyte maintains a stable solvation structure. Therefore, the electrolyte of the embodiments of the present invention can be used in lithium-ion batteries with a cut-off voltage of 4.5V.

[0025] Among them, the mass fraction of the additive lanthanum nitrate in the electrolyte is preferably 0.08% - 0.20%, more preferably 0.10% - 0.15%, and most preferably 0.10%.

[0026] In a feasible implementation, the ether-ester mixed organic solvent is made by mixing ethylene glycol dimethyl ether, fluoroethylene carbonate, and ethyl methyl carbonate in a volume ratio of (0.9 - 1.1):(0.9 - 1.1):(2 - 4). Preferably, the ether-ester mixed organic solvent is made by mixing ethylene glycol dimethyl ether, fluoroethylene carbonate, and ethyl methyl carbonate in a volume ratio of 1:1:3.

[0027] Specifically, the solvent of the lithium-ion battery electrolyte needs to have safety (high flash point), conductivity (low melting point, large dielectric constant), environmental friendliness, cost-effectiveness, compatibility, etc. To obtain a lithium-ion battery with stable operation and good performance, it is necessary to use a mixture of no less than two organic solvents to make up for each other's deficiencies and obtain good battery performance. The melting point of ethylene glycol dimethyl ether is -58°C, the flash point is -2°C, the surface tension is 21.3 dyne / cm, and the dielectric constant is 7.3. The melting point of fluoroethylene carbonate is 18°C, the flash point is 120°C, and the dielectric constant is 102. It is widely used as a film-forming additive in the field of lithium-ion batteries to form a solid electrolyte interface film with low impedance and dense structure and prevent the further decomposition of the electrolyte. In addition, it can also be used as a metal surface treatment agent to improve the anti-corrosion performance of metals. The melting point of ethyl methyl carbonate is -14°C, the surface tension is 25.9 dyne / cm, and the dielectric constant is 2.9. It has both methyl and ethyl structures and combines the characteristics of dimethyl carbonate and diethyl carbonate.

[0028] Considering the above requirements and characteristics, the above three are combined to obtain a mixed organic solvent: the low melting point and low surface tension of ethylene glycol dimethyl ether can ensure the low-temperature fluidity of the solvent system; the high flash point of fluoroethylene carbonate can provide high-temperature stability for the solvent system, the high dielectric constant can ensure the dissociation (solubility) of lithium salts, and the fluorine atoms in the structure are beneficial to the wetting inside the battery cell, and at the same time have the film-forming performance of a solid electrolyte membrane; ethyl methyl carbonate combines the advantages of the current commercial electrolyte linear carbonate solvents (dimethyl carbonate, diethyl carbonate). The ether-ester mixed solvent formed by these three can provide good solute salt dissociation effect, wide liquid temperature range, and interface-assisted film-forming effect, ensure the high stability of the electrolyte system, and at the same time can provide a guiding idea for the development of other mixed solvents. Compared with other solvent systems, these three are primary products in the synthesis process, the acquisition method is relatively simple and the cost is low, and on this basis, good solvent performance can be ensured.

[0029] In a feasible embodiment, the concentration of lithium bis(trifluoromethanesulfonyl)imide or / and lithium bis(fluorosulfonyl)imide in the electrolyte is 0.1 mol / L - 2 mol / L. Preferably, the concentration of lithium bis(trifluoromethanesulfonyl)imide or / and lithium bis(fluorosulfonyl)imide in the electrolyte is 0.9 mol / L - 1.1 mol / L. More preferably, the concentration of lithium bis(trifluoromethanesulfonyl)imide or / and lithium bis(fluorosulfonyl)imide in the electrolyte is 1 mol / L.

[0030] Specifically, lithium bis(trifluoromethanesulfonyl)imide has good electrochemical stability (high-voltage stability, >4.5V vsLi / Li + ) and high conductivity (up to 0.01 S / cm). Even at low temperatures (e.g., -30 °C), the conductivity can still reach 10 -3 S / cm or more. Its flash point (>100 °C) is higher than that of LiPF6 (25 °C) widely used at present, and the operating temperature range is wide. It is insensitive to trace water in the organic system, while LiPF6 widely used at present has high water sensitivity and releases harmful substances such as HF acid and PF5 gas when encountering water, which is not conducive to the stability of the internal components of the battery. Similarly, lithium bis(fluorosulfonyl)imide has strong electrochemical stability, wide temperature range, high solubility, good hydrolysis resistance, and high conductivity (the strong electron-withdrawing property of fluoride ions weakens the coordination effect between anions and cations, thus making Li + have strong mobility). Lithium bis(trifluoromethanesulfonyl)imide and lithium bis(fluorosulfonyl)imide can effectively reduce the interfacial resistance at low temperatures, reduce the capacity loss during the aging process of the battery, and have good effects on stabilizing the electrode interface, improving the battery capacity and battery electrochemical performance, and extending the battery life.

[0031] Given that the cost of the liquid electrolyte of lithium-ion batteries accounts for 15% - 20% of the total cost, among which the solute (lithium salt) accounts for 40% - 50% of the electrolyte cost, the solvent accounts for about 30%, and the additive accounts for 10% - 30%. The addition amounts of the solute and the solvent are relatively large (especially for large-scale energy storage devices, the amount of electrolyte is measured in KG). To control the production cost of lithium-ion batteries, it is necessary to control the amount of the solute. At the same time, to ensure the ionic conductivity of the electrolyte, the addition of lithium salt needs to provide sufficient carrier numbers and high ion mobility. The balance between the two results in the ionic conductivity of the electrolyte. The number of carriers in the electrolyte is determined by the dissociation effect of the solute lithium salt, and the ion mobility is dominated by the intrinsic viscosity of the electrolyte. With the continuous in-depth research on lithium-ion battery electrolytes, it is known that high lithium salt concentration (>3 mol / L) corresponds to high cost, high viscosity, low ion mobility, low wettability of the separator material, and high risk of solvent depletion. Preferably, 1 mol / L lithium salt can provide the best bulk conductivity and cost-effectiveness.

[0032] In a feasible implementation, a preparation method of an electrolyte containing an electrode interface modification additive includes: Perform sufficient dehydration treatment on lanthanum nitrate hexahydrate; Transfer the dried anhydrous lanthanum nitrate into an inert atmosphere glove box for sufficient grinding; In the inert atmosphere glove box, mix ethylene glycol dimethyl ether, fluoroethylene carbonate, and ethyl methyl carbonate to prepare an ether-ester mixed organic solvent; Add lithium bis(trifluoromethanesulfonyl)imide or / and lithium bis(fluorosulfonyl)imide to the ether-ester mixed solvent to prepare a mixed organic electrolyte. Stir at room temperature. During the stirring process, slowly add the ground anhydrous lanthanum nitrate to the mixed organic electrolyte. After sufficient stirring, let it stand to obtain an electrolyte containing an electrode interface modification additive.

[0033] In the second aspect of the embodiments of the present invention, a lithium-ion battery operating at a cut-off voltage of 4.5V is provided, which includes a positive electrode material, a negative electrode material, and the above-mentioned electrolyte. Among them, the positive electrode material is selected from any one of nickel-cobalt-manganese ternary positive electrodes or lithium nickel manganate; the negative electrode material is selected from any one of graphite or lithium metal.

[0034] The electrolyte system provided by the embodiments of the present invention has good compatibility. The lithium-ion battery supported thereby can achieve good cycle performance and good safety. The first-cycle Coulombic efficiency of the battery at 0.2C can reach 91.9%.

[0035] Without special instructions, in the embodiments of the present invention, the preparation method of the ternary nickel-cobalt-manganese positive electrode sheet is as follows: The positive electrode active material LiNi 0.6 Co 0.2 Mn 0.2 O2, conductive agent Super P, and binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 7:1:1, placed in an agate mortar and ground for more than 40 minutes, and then added to N-methyl-2-pyrrolidone (NMP) for magnetic stirring for 4 hours to make a slurry. The slurry is evenly coated on the positive electrode aluminum current collector, dried in a vacuum drying oven at 80°C for 12 hours, and then sliced to obtain the positive electrode sheet.

[0036] The positive electrode case, negative electrode case, stainless steel gasket, and spring piece are purchased from Dongguan Kelude Experimental Equipment Technology Co., Ltd., with the model CR2032. The positive electrode current collector aluminum foil, negative electrode current collector copper foil, negative electrode lithium metal sheet, and separator (model Celgard2500) are purchased from Dongguan Kelude Experimental Equipment Technology Co., Ltd.

[0037] Example 1 An electrolyte containing an electrode interface modification additive and its preparation method (I) Electrolyte The conductive salt is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and the concentration of the conductive salt is 1 mol / L; The additive is lanthanum nitrate with a mass fraction of 0.10%; The solvent is an ether-ester mixed organic solvent, specifically ethylene glycol dimethyl ether (DME), fluoroethylene carbonate (FEC), and ethyl methyl carbonate (EMC) with a volume ratio of 1:1:3.

[0038] (II) Preparation method It includes the following steps: (1) 10 g of lanthanum nitrate hexahydrate (La(NO3)3•6H2O) is fully dehydrated and dried in a vacuum drying oven at 40 °C for 8 h; (2) The dried anhydrous lanthanum nitrate (La(NO3)3) is transferred into an inert atmosphere glove box and ground with an agate mortar for more than 30 minutes; (3) In the inert atmosphere glove box, DME, FEC, and EMC are respectively prepared into an ether-ester mixed organic solvent according to a volume ratio of 1:1:3; (4) 2.871 g of the conductive salt LiTFSI is added to a certain amount of the ether-ester mixed organic solvent, fixed volume to 10 mL and weighed. The concentration of the conductive salt is 1 mol / L, stirred at room temperature for 6 h. During the stirring process, 0.1% La(NO3)3 by mass is slowly added thereto. After sufficient stirring, it is left standing for the next step; (5) The standing mixed solution is used as the electrolyte to assemble NCM622||Li half-cells, Li||Al cells, Li||Cu cells, and Li||Li symmetric cells.

[0039] (III) Testing method Battery performance test: The assembled NCM622||Li battery is left standing for 12 hours, and the cycle performance test is carried out using a Blue Power battery test system. First, it is charged at a constant voltage of 3.9 V to eliminate stray current until the current rate is less than 0.05 C (1 C = 200 mAh / g), and then charged and discharged at a rate of (0.2 C) for 50 cycles, and the initial Coulomb efficiency is calculated. The initial Coulomb efficiency is equal to the initial discharge specific capacity divided by the initial charge specific capacity.

[0040] The assembled Li||Li symmetric battery is left standing for 12 hours, and the time-voltage curve test is carried out using a Blue Power battery test system, with constant current (0.5 mA / cm 2 ) charged and discharged for 200 cycles, the charging and discharging step time is set to 2 hours, and then charged and discharged at a constant current of (1 mA / cm 2 ) for 300 cycles, and the charging and discharging step time is set to 1 hour.

[0041] The assembled Li||Cu battery was allowed to stand for 12 hours, and the time-voltage curve test was carried out using a Blue Power battery test system. A constant current of 0.5 mA / cm 2 was used to discharge first and then charge for 1 cycle and then discharge once. The upper limit of the charging voltage was 1 V, and the discharge step time was set to 10 hours. Then, a constant current of 0.5 mA / cm 2 was used to discharge first and then charge for 10 cycles, and the time for this charge-discharge step was set to 2 hours.

[0042] Electrochemical behavior test: The assembled Li||Al battery was allowed to stand for 12 hours, and the alternating current impedance, cyclic voltammetry curve, and time-current curve at different voltages of the working electrode Al were tested using a Chenhua electrochemical workstation CHI760E. The cyclic voltammetry scan rate was 10 mV / s; the alternating current impedance frequency range was 0.01 Hz - 10 5 Hz, and the amplitude was 0.5 mV; for the time-current curve, voltages of 3.8 V, 4.0 V, 4.2 V, and 4.5 V were used, and the polarization times were 1 hour respectively.

[0043] Example 2 The difference from Example 1 was that the mass fraction of La(NO3)3 was 0.05%.

[0044] Example 3 The difference from Example 1 was that the mass fraction of La(NO3)3 was 0.15%.

[0045] Example 4 The difference from Example 1 was that the mass fraction of La(NO3)3 was 0.20%.

[0046] Example 5 The difference from Example 1 was that the mass fraction of La(NO3)3 was 0.30%.

[0047] Example 6 The difference from Example 1 was that the solvent used was DME, FEC, and EMC in a volume ratio of 1:1:2.

[0048] Example 7 The difference from Example 1 was that the solvent used was DME, FEC, and EMC in a volume ratio of 1:1:4.

[0049] Comparative Example 1 The difference from Example 1 was that La(NO3)3 was not added.

[0050] Comparative Example 2 The difference from Example 1 was that the mass fraction of La(NO3)3 was 0.35%.

[0051] Comparative Example 3 It is different from Example 1 in that the mass fraction of La(NO3)3 is 0.40%.

[0052] Comparative Example 4 It is different from Example 1 in that the solvent is DME.

[0053] Comparative Example 5 It is different from Example 1 in that the solvent is DME and FEC, and the volume ratio of the two is 1:1.

[0054] Comparative Example 6 It is different from Example 1 in that the solvent is DME, FEC and EMC in a volume ratio of 1:1:6.

[0055] Comparative Example 7 It is different from Example 1 in that the solvent is DME, FEC and EMC in a volume ratio of 1:1:8.

[0056] Table 1 Performance of electrolytes and lithium-ion batteries based on Examples 1-7 and Comparative Examples 1-7

[0057] Results and Discussion According to Figure 1 , considering that La(NO3)3 is soluble in organic solvents and can dissociate into La 3+ and NO3 - , both are beneficial to the interfacial modification of the electrode. In the present invention, La(NO3)3 is used as an electrolyte additive, which helps to construct a stable and uniform positive and negative electrode interface. The standard electrode potential of La / La 3+ is 0.66V vs Li / Li + , and the reduction potentials of the possible La 3+ -containing substances LaF3 and La(NO3)3 are higher than Li / Li + . Therefore, La 3+ can be preferentially deposited in the lithium-ion battery to pre-clean the surface of the negative electrode for Li + , instead of consuming Li + during the activation process to reduce the substances on the surface of the negative copper current collector, prevent the loss of active lithium and achieve uniform deposition of lithium. The existence of LaF3 depends on the anion exchange in the solvation structure of LiTFSI and La(NO3)3 to form La(TFSI)3 and defluorination. According to thermodynamic calculations, the conversion of LaF3 and LiNO3 to LiF and La(NO3)3 can occur spontaneously, which means that LaF3 basically does not exist in the chemical environment, thus avoiding the superposition of electrode side reactions.

[0058] Nitrate anion (NO3- has an electron donor number (DN) of 21.1 kcal / mol, which is much higher than that of TFSI - with a DN number of 5.4 kcal / mol. Therefore, NO3 - can capture the Al that undergoes dissolution corrosion due to the high-voltage operation of the LiTFSI battery 3+ and achieve anion-cation coordination. According to the hard and soft acid-base theory, aluminum nitrate Al(NO3)3 is more stable than Al(TFSI)3 and then forms a precipitate on the Al surface, interrupting the contact between LiTFSI in the electrolyte and the Al matrix, thereby achieving passivation protection of the positive electrode interface. From Figure 2 the cyclic voltammetry curve, it can be seen that the oxidation potential of Al during the positive sweep of the voltage in Li||Al of Example 1 (5.85 V vs Li / Li + ) is much greater than that of Comparative Example 1 (3.6 V vs Li / Li + ) and Comparative Example 5 (5 V vs Li / Li + ). At the same potential, the current density of Example 1 is smaller than that of Comparative Example 1 and Comparative Example 5, that is, the anodic dissolution or electrolyte oxidation reaction rate is slower. Although the anodic dissolution current density of Comparative Example 5 is reduced compared to Comparative Example 1, there is still serious dissolution corrosion phenomenon, and the current density increases (the increase amplitude is also large) when the scanning potential changes from positive sweep to negative sweep in Comparative Example 1 and Comparative Example 5. This indicates that the high potential stimulates the rapid dissolution of Al and the decomposition reaction of the electrolyte. Therefore, even the continuously increasing Al dissolution current during the negative sweep cannot be offset by the reverse current of the external circuit.

[0059] From the time-current curve Figure 3 it can be seen that the La(NO3)3 additive in Example 1 effectively reduces the oxidation current of Al at a high voltage of 4.5 V. The current density corresponding to the stepped potential sweep of Comparative Example 1 increases with the increase of the potential, which means the stimulation of the high voltage on the anodic dissolution of Al. Cathodic reduction currents (current density less than 0) appear in Example 1 and Comparative Example 5 at different potentials, which means that the voltage does not trigger the "electron loss" reaction on the Al electrode itself (no Al→Al 3+ occurs). This result is consistent with the curve law of Figure 2 : The Al electrode is in a stable state at this potential (3.8 V, 4.0 V, 4.2 V, 4.5 V vs Li / Li + ). The absolute value of the current density of Comparative Example 5 is less than that of Example 1 at 4.5 V vsLi / Li + , which also means that at a higher voltage (5 V), Comparative Example 5 cannot inhibit the anodic dissolution of Al (the current will show an anodic current and gradually increase).

[0060] From the AC impedance Figure 4It can be seen that Example 1 has a larger interfacial reaction resistance compared to Comparative Example 1. By calculating the modulus values of the real part (Z’) and imaginary part (Z’’) of the AC impedance of Example 1 and Comparative Example 1, it can also be known that the impedance modulus value of Comparative Example 1 is smaller than that of Example 1. The surface of Al in Example 1 has good passivation ability (low ionic conductivity, and Al is not easily converted to Al 3+ ) which can effectively inhibit the dissolution and corrosion of the substrate.

[0061] From Figure 5 it can be seen that after the positive current collector of the lithium-ion battery in Example 1 was subjected to a variety of high-voltage polarization treatments, no change occurred on the surface. However, on the surface of Comparative Example 1, micron-sized pitting corrosion pits appeared, and the corrosion points were concentrated and continuous (as indicated by the white arrows in the figure). This feature indicates that the positive Al current collector in the LiTFSI electrolyte of Comparative Example 1 will continuously dissolve and perforate, resulting in irreversible capacity loss in NCM622||Li. On the surface of Example 4, more new substances appeared. At the same time, referring to the time-voltage curve of the symmetric battery of Example 4 Figure 6 , it can be seen that at this time, La(NO3)3 is in an excessive state, which is not conducive to the optimization of the interfacial performance. However, Example 4 still has an advantage compared to Comparative Example 4. Comparative Example 4 shows obvious polarization phenomena at 1 mA. Such polarization is caused by side reactions at the electrode-electrolyte interface during the repeated lithium deposition-stripping process. According to Figure 6 , for Comparative Example 3, the voltage is relatively stable during charge and discharge at 0.5 mA / cm 2 , but serious polarization phenomena occur to the electrode during charge and discharge at 1 mA / cm 2 . The ohmic impedance and charge transfer resistance inside the symmetric battery increase sharply, resulting in the battery voltage increasing to 0.40 V. The battery electrolyte is already in a depleted state and the battery is about to open circuit due to excessive voltage. Comparative Example 4 shows serious polarization phenomena (the voltage value fluctuates greatly (exceeding ±0.5 V), that is, the overpotential of the electrode reaction is extremely large) even during charge and discharge at 0.5 mA / cm 2 , and finally the battery open circuit cycle terminates. Similarly, during charge and discharge at 0.5 mA / cm 2 for Example 4, a large overpotential (exceeding ±0.5 V) also appears. However, as the charge and discharge process progresses, the voltage gradually decreases and tends to be stable. Subsequently, the voltage is still in a relatively stable state during charge and discharge at 1 mA / cm 2 . Nevertheless, the overpotential of Example 4 will still cause an open circuit phenomenon on the surface of the lithium metal electrode, which is not conducive to the long-term cycling of the battery.

[0062] According to Figure 7 a, the NCM622||Li battery of Example 1 shows a first-cycle Coulombic efficiency of 91.90% at 0.2C, and the first-cycle discharge capacity is 214.5 mAh / g. In the Li||Al battery, the macroscopic morphology of the negative lithium surface is as Figure 7As shown in Fig. b, the surface of the lithium anode in Example 1 has a metallic luster, while black substances appear on the lithium anode in Comparative Example 1, indicating that the dissolution of the positive current collector Al in the electrolyte without additives will cause uneven deposition of lithium and side reactions of the electrolyte.

[0063] Based on the performance of the batteries and the oxidation onset potential of the positive Al current collector in the electrolytes of Examples 1-7 and Comparative Examples 1-7 are shown in Table 1. The first-cycle discharge capacities of Examples 1-7 are between 208 mAh / g and 216 mAh / g, and the first-cycle Coulombic efficiency is above 80.00% (superior to Comparative Examples 2-7). The oxidation onset potential of Al in Li||Al is greater than 5V except for Example 2 (4.5V vs Li / Li + ). The first-cycle deposition overpotential of metals in Li||Cu of Examples 1-7 increases with the increase of the addition amount of La(NO3)3. This potential value corresponds to the Li + (Example 2) or La 3+ deposition potential. Li + →Li 0 gaining electrons deposits on the surface of La metal (as can be seen from Figure 1 , La 3+ →La 0 is prior to the reduction of Li + ) corresponding to a large overpotential. Normally, the smaller the deposition onset overpotential of Li + , the more favorable it is for the negative electrode Li + to gain electrons and become Li 0 . In Example 2, only 0.05% of La(NO3)3 is added. The first-cycle Coulombic efficiency of the battery and the oxidation onset potential of Al (4.5V vs Li / Li + ) indicate that the dissolution of Al reduces the interface stability and the overall performance of the battery. Even if the deposition onset potential of the metal is the smallest, it cannot ensure the normal long-term operation of the battery. Therefore, it is necessary to balance between the deposition onset overpotential of the metal and the oxidation onset potential of Al, ensuring both the stability of the positive Al and a smaller energy barrier for the deposition of the negative Li + . Thus, the appropriate addition of La(NO3)3, even with a relatively high deposition overpotential, reasonably provides the preferential deposition of La, which can pre-clean the surface of the negative electrode for Li + , replace the Li + consumed during the activation process to reduce the substances on the surface of the negative copper current collector, prevent the loss of active lithium, and achieve uniform deposition of lithium, specifically manifested as a high Coulombic efficiency and a neat morphology of the Li negative electrode.

[0064] The initial charge-discharge capacities of Comparative Examples 1-7 ranged from 153 mAh / g to 222 mAh / g. The low initial charge-discharge capacities of Comparative Examples 2-3 were caused by the excess of La(NO3)3. The initial Coulombic efficiencies of Comparative Examples 1-7 were below 80.00% (except for Comparative Example 1), and the excessive initial deposition overpotentials of metals in Li||Cu were not advantageous. The initial oxidation onset potentials of Al in Li||Al showed different values. The Al in Comparative Example 1 underwent anodic dissolution at 3.6 V, the Al in Comparative Examples 4-5 underwent anodic dissolution above 4.5 V, and the Al in Comparative Examples 2-3 and Comparative Examples 6-7 could remain stable below 5 V. The low initial Coulombic efficiencies of Comparative Examples 2-7 were caused by the anodic dissolution of Al at the positive electrode during charging or the too high deposition energy barrier of Li at the negative electrode. The charging capacity contributed by Al→Al 3+ was irreversible, so the resulting capacity loss was manifested as a low discharge capacity. The battery performance of Comparative Example 1 was superior, but its initial oxidation onset potential of Al was 3.6 V vs Li / Li + , which means that the dissolution of Al started before reaching the charging platform of NCM622 ( Figure 7 the charging platform in which was 4.0 V vs Li / Li + ), and this behavior was extremely unfavorable to the stable operation of the battery. According to the cask effect, obviously Example 1 had obvious advantages and had a good effect on improving the electrode interface stability and optimizing the battery performance.

[0065] In summary, the present invention proposes an electrolyte and a lithium-ion battery containing an electrode interface modification additive. Among them, the vacuum-dried La(NO3)3 was added to the ether-ester mixed organic solvent of DME, FEC, and EMC (volume ratio 1:1:3). The addition of 0.1% mass fraction of La(NO3)3 could increase the 0.2C initial Coulombic efficiency of the NCM622||Li battery to 91.90%. The initial charge-discharge capacity was 214.5 mAh / g, and Al(NO3)3 precipitates could form on the surface of the positive electrode current collector Al, and the Al oxidation potential increased to above 5 V. In the Li||Li and Li||Al batteries, preferential deposition of La appeared at the negative electrode 3+ , avoiding the loss of active lithium (the initial Coulombic efficiency was significantly improved), and showing uniform deposition of lithium and inhibition of side reactions. The lithium-ion battery prepared by using the electrolyte containing the electrode interface modification additive of the present invention showed positive and negative electrode compatibility, and thus the interface regulation of the positive and negative electrodes could be realized to ensure the stable operation of the battery.

[0066] Those skilled in the art can easily understand that, on the premise of no conflict, the above-mentioned advantageous ways can be freely combined and superimposed. The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present application shall be included within the protection scope of the present application. The above is only the preferred implementation manner of the present application. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present application, several improvements and modifications can still be made, and these improvements and modifications should also be regarded as within the protection scope of the present application.

Claims

1. An electrolyte containing an electrode interface modification additive, characterized in that, Comprising: The conductive salt is lithium bis(trifluoromethanesulfonyl)imide or / and lithium bis(fluorosulfonyl)imide; The additive is lanthanum nitrate, and the mass fraction of the lanthanum nitrate in the electrolyte is 0.05% - 0.30%; The solvent is an ether-ester mixed organic solvent.

2. The electrolyte containing the electrode interface modification additive according to claim 1, wherein, The mass fraction of the lanthanum nitrate in the electrolyte is 0.08% - 0.20%.

3. The electrolyte containing an electrode interface modification additive according to claim 2, characterized in that, The mass fraction of the lanthanum nitrate in the electrolyte is 0.10% - 0.15%.

4. The electrolyte containing an electrode interface modification additive according to claim 1, wherein The ether-ester mixed organic solvent is prepared by mixing ethylene glycol dimethyl ether, vinylene carbonate, and ethyl methyl carbonate in a volume ratio of (0.9 - 1.1):(0.9 - 1.1):(2 - 4).

5. The electrolyte containing an electrode interface modification additive according to claim 4, characterized in that, The ether-ester mixed organic solvent is prepared by mixing ethylene glycol dimethyl ether, vinylene carbonate, and ethyl methyl carbonate in a volume ratio of 1:1:

3.

6. The electrolyte containing an electrode interface modification additive according to claim 1, wherein, The concentration of the lithium bis(trifluoromethanesulfonyl)imide or / and lithium bis(fluorosulfonyl)imide in the electrolyte is 0.1 mol / L - 2 mol / L.

7. The electrolyte containing the electrode interface modification additive according to claim 6, characterized in that, The concentration of the lithium bis(trifluoromethanesulfonyl)imide or / and lithium bis(fluorosulfonyl)imide in the electrolyte is 0.9 mol / L - 1.1 mol / L.

8. The electrolyte containing an electrode interface modification additive according to claim 1, wherein The electrolyte is used for a lithium-ion battery with a cut-off voltage of 4.5 V.

9. The electrolyte containing the electrode interface modification additive according to any one of claims 1-8, characterized in that, The preparation method of the electrolyte comprises: Subjecting lanthanum nitrate hexahydrate to sufficient dehydration treatment; Transferring the dried anhydrous lanthanum nitrate into an inert atmosphere glove box for sufficient grinding; In the inert atmosphere glove box, mixing ethylene glycol dimethyl ether, vinylene carbonate, and ethyl methyl carbonate to prepare the ether-ester mixed organic solvent; Adding the lithium bis(trifluoromethanesulfonyl)imide or / and lithium bis(fluorosulfonyl)imide to the ether-ester mixed solvent to prepare a mixed organic electrolyte, stirring at room temperature, and during the stirring process, slowly adding the ground anhydrous lanthanum nitrate to the mixed organic electrolyte, fully stirring and then standing to obtain the electrolyte containing the electrode interface modification additive.

10. A lithium-ion battery, characterized in that, The lithium-ion battery operates at a cut-off voltage of 4.5 V, and the lithium-ion battery comprises a positive electrode material, a negative electrode material, and the electrolyte according to any one of claims 1 - 9; Wherein, the positive electrode material is selected from any one of nickel-cobalt-manganese ternary positive electrode or lithium nickel manganate; the negative electrode material is selected from any one of graphite or lithium metal.

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