Preparation method and application of low-temperature electrolyte for NCM92 positive electrode

Abstract

The invention relates to a low-temperature electrolyte modification technology suitable for an NCM92 ultrahigh-nickel ternary positive electrode material, which adopts a specific experimental method that the solvent ratio is optimized by introducing a linear carboxylic ester solvent with low viscosity and low freezing point, and the low-temperature dissociation performance is improved by adopting a double-lithium salt composite system design. And finally, adding a film-forming additive to optimize an interfacial film structure and reduce low-temperature impedance. According to the method, the low-temperature ion transmission efficiency, the low-temperature discharge capacity and the cycling stability of the battery are remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion batteries, and particularly relates to a low-temperature electrolyte suitable for ultra-high nickel ternary cathodes, and its preparation and application. Background Art

[0002] With the surge in demand for high-energy-density lithium-ion batteries in new energy vehicles and energy storage systems, ultra-high nickel ternary cathode materials (LiNi x Co y Mn z O 2 , x≥0.9) have become a research hotspot due to their theoretical capacity of over 210 mAh / g. However, their electrochemical performance deteriorates sharply at low temperatures (below -20°C), mainly manifested as a sharp drop in discharge capacity (less than 50% of room temperature), a significant increase in polarization voltage (>200 mV), and a shortening of cycle life (capacity retention rate <70%). At the same time, traditional electrolyte systems (such as LiPF 6 / EC+DMC) further exacerbate the deterioration of battery performance at low temperatures due to the increase in solvent viscosity and the decrease in lithium salt dissociation degree. Therefore, to develop a suitable low-temperature electrolyte for ultra-high nickel ternary cathode materials, new ideas are needed to break through the existing technical bottlenecks.

[0003] Ultra-high nickel materials (nickel content ≥90%) have poor structural stability at high voltages, and lattice oxygen release, microcrack generation, and interfacial phase transformation are prone to occur during the cycling process, and these problems are more significant at low temperatures. At low temperatures, the diffusion kinetics of lithium ions in the cathode material is hindered, resulting in increased polarization, leading to lithium metal precipitation and capacity loss. In addition, due to the stress concentration at the grain boundaries of polycrystalline ternary cathode materials, cracks are prone to occur during low-temperature charge and discharge, accelerating electrolyte penetration and triggering side reactions. Although the single-crystallization process (such as two high-temperature sinterings combined with ball milling and crushing) can reduce grain boundary defects, its preparation cost is high and its compatibility requirements for electrolytes are higher. The main problems of existing electrolyte systems at low temperatures include: high freezing point of the solvent system: the viscosity of traditional carbonate solvents (such as EC, DMC) increases significantly at low temperatures, resulting in a decrease in ionic conductivity (such as the conductivity at -20°C is only 20% of room temperature). Low lithium salt dissociation degree: LiPF 6 is prone to crystallization at low temperatures, and the intermolecular force with solvent molecules increases, inhibiting the migration of lithium ions. Increased interfacial film impedance: at low temperatures, the SEI film on the negative electrode and the CEI film on the positive electrode thicken, hindering ion transport, and at the same time, the oxidation and decomposition of the electrolyte are aggravated, generating high-impedance by-products.

[0004] To address the above problems, the research on the modification of the low-temperature electrolyte designed in this invention mainly focuses on the following aspects: (1) Co-solvent modification: Optimize the solvent ratio by introducing solvents with low viscosity and low freezing point (such as linear carboxylic esters, fluorinated carbonates). For example, the combination of linear carboxylic esters (such as EMC, DMC) and fluorinated solvents (such as FEC) can reduce the freezing point of the system and improve the oxidation resistance. (2) Lithium salt modification: Use composite lithium salts or new lithium salts to improve the low-temperature dissociation performance. Boron-containing lithium salts (such as LiDFOB) have become a research hotspot for low-temperature electrolytes due to their low melting point and high dissociation degree, but they are costly and sensitive to moisture. (3) Functional additives: Optimize the interfacial film structure through film-forming additives (such as VC, FEC) to reduce the low-temperature impedance.

[0005] In summary, developing an electrolyte system that takes into account cycle stability, low-temperature ion transport efficiency, and interfacial compatibility is the key to breaking through the low-temperature performance bottleneck of ultra-high nickel ternary batteries. This patent realizes a double improvement in the low-temperature performance of the electrolyte and the compatibility with the cathode material through the co-solvent ratio optimization (linear carboxylic ester co-solubility) and the synergistic design of composite lithium salts (LiPF 6 +LiDFOB), combined with the development of film-forming additives (FEC). Summary of the Invention

[0006] To address the above problems or deficiencies, in order to solve the problems such as high freezing point of the solvent system, low dissociation degree of lithium salts, and increased interfacial film impedance in the nine-series high-nickel materials, a low-temperature electrolyte system is proposed. Through the synergistic ratio optimization of carboxylic ester co-solvents and carbonate solvents, the design of a dual-lithium salt composite system, and the interfacial regulation of film-forming additives, the low-temperature ion transport efficiency, interfacial stability, and cycle life of the battery are significantly improved. To address the above problems or deficiencies, in order to solve the problems such as high freezing point of the solvent system, low dissociation degree of lithium salts, and increased interfacial film impedance in the nine-series high-nickel materials, a low-temperature electrolyte system is proposed. Through the synergistic ratio optimization of carboxylic ester co-solvents and carbonate solvents, the design of a dual-lithium salt composite system, and the interfacial regulation of film-forming additives, the low-temperature ion transport efficiency, interfacial stability, and cycle life of the battery are significantly improved.

[0007] To achieve the above object, the present invention adopts the following technical solutions: A low-temperature electrolyte that significantly improves the low-temperature performance of the NCM92 ultra-high nickel ternary cathode, comprising a lithium salt, an organic solvent, and an additive, and the concentration of the electrolyte is 1-3 mol / L.

[0008] The salt is a lithium salt, a sodium salt, and / or a potassium salt. The organic solvent includes carbonates, carboxylic esters, and / or ethers, and the additive is a fluorinated carbonate.

[0009] Further, the lithium salt is: lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4), lithium perchlorate (LiClO 4 ), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), lithium bis(fluorosulfonyl)imide (LiFSI) and / or lithium bis(trifluoromethylsulfonyl)imide (LiTFSI).

[0010] Furthermore, the sodium salt is: sodium hexafluorophosphate (NaPF 6 ), sodium tetrafluoroborate (NaBF 4 ), sodium perchlorate (NaClO 4 ), sodium bis(oxalato)borate (NaBOB), sodium difluoro(oxalato)borate (NaDFOB), sodium bis(fluorosulfonyl)imide (NaFSI) and / or sodium bis(trifluoromethylsulfonyl)imide (NaTFSI).

[0011] Furthermore, the potassium salt is: potassium hexafluorophosphate (KPF 6 ), potassium tetrafluoroborate (KBF 4 ), potassium perchlorate (KClO 4 ), potassium bis(oxalato)borate (KBOB), potassium difluoro(oxalato)borate (KDFOB), potassium bis(fluorosulfonyl)imide (KFSI) and / or potassium bis(trifluoromethylsulfonyl)imide (KTFSI).

[0012] Furthermore, the carbonates are: ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC) and / or vinylene carbonate (VC).

[0013] Furthermore, the carboxylates are: γ-butyrolactone (GBL), methyl acetate (MA), ethyl acetate (EA), ethyl butyrate (EB), propyl acetate (PA), methyl propionate (MP) and / or ethyl propionate (EP).

[0014] Furthermore, the ethers are: 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (TEE), diethylene glycol dimethyl ether (DEGDME), triethylene glycol dimethyl ether (TriEGDME), tetraethylene glycol dimethyl ether (TeEGDME), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF) and / or 1,3-dioxolane (DOL).

[0015] Furthermore, the fluorinated carbonates are: fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoroethylene carbonate (TFEC), fluoropropylene carbonate (FPC), difluoropropylene carbonate (DFPC), trifluoropropylene carbonate (TFPC), fluorodimethyl carbonate (FDMC), fluoroethyl methyl carbonate (FEMC), fluorodiethyl carbonate (FDEC).

[0016] In view of the problems of high low-temperature viscosity and high freezing point of traditional carbonate solvents (such as EC and DMC), the present invention adopts a composite solvent of carboxylic acid ester and carbonate. The specific ratio is as follows: the proportion of carboxylic acid ester solvent is 20-40 vol%, and its low melting point (such as the melting point of EA is -83.6 °C) and low viscosity characteristics can significantly reduce the freezing point of the electrolyte and improve the conductivity below -40 °C. The proportion of carbonate solvent is 60-80 vol%, and its high dielectric constant (such as the dielectric constant of EC is 89.6) can enhance the dissociation of lithium salts. At the same time, it forms a eutectic effect with carboxylic acid esters to inhibit the low-temperature crystallization risk of single solvents. Fluorinated carbonate is preferably used as an additive component, and its fluorine atoms can improve the antioxidant property of the solvent (oxidation potential ≥ 5.0 V vs. Li + / Li), meeting the voltage (≥ 4.2 V) requirements of ultra-high nickel materials.

[0017] The present invention forms a "solvent-lithium salt-additive" three-in-one low-temperature electrolyte solution by reducing viscosity and freezing point through a carboxylic acid ester / carbonate co-solvent system, enhancing dissociation degree and interface stability through double lithium salt composites, and optimizing the SEI / CEI film structure with film-forming additives, providing technical support for the application of ultra-high nickel ternary batteries in extreme environments. Detailed implementation mode Example

[0019] In a glove box filled with argon, use a pipette to take ethylene carbonate (EC) with a volume fraction of 18%, diethyl carbonate (DEC) with a volume fraction of 42%, and ethyl acetate (EA) with a volume fraction of 40%. Finally, add vinylene carbonate fluoride (FEC) with a mass fraction of 2% and mix evenly. Then, add lithium hexafluorophosphate (LiPF 6 6) and lithium difluorooxalate borate (LiDFOB) with a molar ratio of 1:1 to the mixed solvent. After it is completely dissolved, a lithium salt electrolyte with a concentration of 1 mol / L is prepared. Example

[0020] In a glove box filled with argon, use a pipette to take ethylene carbonate (EC) with a volume fraction of 18%, diethyl carbonate (DEC) with a volume fraction of 42%, and ethyl acetate (EA) with a volume fraction of 40%. Finally, add vinylene carbonate fluoride (FEC) with a mass fraction of 2% and mix evenly. Then, add lithium hexafluorophosphate (LiPF 6 6) and lithium difluorooxalate borate (LiDFOB) with a molar ratio of 1:1 to the mixed solvent. After it is completely dissolved, a lithium salt electrolyte with a concentration of 2 mol / L is prepared. Example

[0021] In a glove box filled with argon, use a pipette to take ethylene carbonate (EC) with a volume fraction of 18%, diethyl carbonate (DEC) with a volume fraction of 42%, and ethyl acetate (EA) with a volume fraction of 40%. Finally, add fluoroethylene carbonate (FEC) with a mass fraction of 2% and mix evenly. Then, add lithium hexafluorophosphate (LiPF 6 ) and lithium difluoro(oxalato)borate (LiDFOB) with a molar ratio of 1:1 to the mixed solvent. After it is completely dissolved, a lithium salt electrolyte with a concentration of 3 mol / L is prepared. Example

[0022] In a glove box filled with argon, use a pipette to take ethylene carbonate (EC) with a volume fraction of 18%, diethyl carbonate (DEC) with a volume fraction of 42%, and ethyl propionate (EP) with a volume fraction of 40%. Finally, add fluoroethylene carbonate (FEC) with a mass fraction of 2% and mix evenly. Then, add lithium hexafluorophosphate (LiPF 6 ) and lithium difluoro(oxalato)borate (LiDFOB) with a molar ratio of 1:1 to the mixed solvent. After it is completely dissolved, a lithium salt electrolyte with a concentration of 2 mol / L is prepared.

[0023] In a glove box filled with argon, use a pipette to take ethylene carbonate (EC) with a volume fraction of 30% and diethyl carbonate (DEC) with a volume fraction of 70% and mix evenly. Then, add lithium hexafluorophosphate (LiPF 6 ) to the mixed solvent. After it is completely dissolved, a lithium salt electrolyte with a concentration of 1 mol / L is prepared.

[0024] The preparation and test conditions of the electrodes of the present invention are described below: Preparation of the electrode: The high-nickel ternary cathode material LiNi 0.92 Co 0.05 Mn 0.03 O 2(NCM92), conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) were weighed according to a mass ratio of 9:0.5:0.5 and placed in an agate mortar. Then, the above mixture was thoroughly ground evenly for 30 min. After the powder was ground fine enough, N-methylpyrrolidone (NMP) was added to the mortar and ground again until the slurry was uniform, and NMP was added according to the state of the slurry. After that, the slurry was evenly blade-coated onto carbon-coated aluminum foil with a 200 μm blade on an electrode coater and transferred to an 80 °C constant-temperature air-circulating drying oven for drying for 4 h. The dried electrode sheet was taken out, punched into circular sheets with a diameter of 14 mm using a manual slicing machine, and the electrode circular sheets were transferred to a vacuum drying oven at 100 °C for drying for 12 h. After being completely dried, the mass of the circular sheets was weighed on an electronic analytical balance and recorded, and finally, they were placed in an 80 °C vacuum drying oven for standby. In a glove box, using a CR2025 button cell case, the positive electrode sheet was placed in the center of the positive electrode case, 20 μL of electrolyte was dropped, a Celgard 2400 separator was placed, and 20 μL of electrolyte was dropped again. Then, a lithium metal sheet (0.45 mm), a gasket, a spring sheet, and a negative electrode case were successively used, and the button cell was encapsulated with a battery sealer.

[0025] Test conditions: The assembled battery was left to stand in a battery compartment (constant temperature 25 °C) for more than 4 h to wait for the electrolyte to fully infiltrate the electrodes. A LAND CT2001A battery test system was used for constant current charge and discharge tests. The discharge current density was 1C (1C = 200 mAh / g), and the charge and discharge voltage range was 3 - 4.3V. For low-temperature tests, the pre-formulated button cells were placed in a GDJ-150 low-temperature oven for constant current discharge tests. The temperature range was -20 °C to -60 °C, the test current rate was 0.1C (1C = 200 mAh / g), and the voltage range was 2 - 4.3V.

[0026] After assembling the above examples and blank control samples into batteries for charge and discharge tests, the test results are shown in Table 1. Under the same test conditions, Examples 1, 2, and 4 using a carboxylic ester / carbonate co-solvent system, a double lithium salt composite, and an additive for forming a film had a much higher capacity retention rate after 250 cycles than Control Group 1. The low-temperature test results are shown in Table 2. The low-temperature discharge test also showed that optimizing the electrolyte greatly improved the low-temperature discharge capacity of the NCM92 material. These results prove that the electrolyte designed in the present invention can well improve the low-temperature performance and cycle life of the NCM92 positive electrode.

[0027] Table 1: Discharge specific capacity of examples and control groups at a 1C rate (25 °C) Number of turns mAh / g 1st 100th 250th Example 1 204.17 197.67 184.28 Example 2 211.9 207.18 200.28 Example 3 193.5 190.73 140.4 Example 4 206.46 197.33 182.32 Control group 1 215.11 198.39 168.3 Table 2: Discharge capacity of examples and control groups at different temperatures Temperature mAh / g 25℃ -20℃ -40℃ -60℃ Control group 1 212.28 183.4 129.7 25.1 Example 2 206.46 191.1 155.2 140.2 Example 4 206.88 191.3 156.4 127 Figure 1 Cyclic test diagram for the example group and the control group Figure 2 Low-temperature discharge test diagram for Example 2 Figure 3 Low-temperature discharge test diagram for Example 4 Figure 4 Low-temperature discharge test diagram for Control Group 1

Claims

1. Preparation of electrolyte: The components of low-temperature electrolyte include lithium salt, organic solvent and additive, and the concentration of electrolyte is 1-3 mol / L. The salt is lithium salt, sodium salt and / or potassium salt. The organic solvent includes carbonate, carboxylic acid ester and / or ether, and the additive is fluorocarbonate.

2. The electrolyte solvent adopts a composite solvent of carboxylate and carbonate, and the specific ratio is: carboxylate solvent accounts for 20~40vol%, carbonate solvent accounts for 60~80vol%, and the film-forming additive is preferably fluorinated carbonate.

3. Battery test conditions: Use LANDCT2001A battery test system for constant current charge and discharge test, discharge current density is 1C (1C=200mAh / g), charge and discharge voltage range is 3-4.3V. Low temperature test is to place the pre-formed button cell in GDJ-150 low temperature oven for constant current discharge test, the temperature range is -20℃~-60℃, the test current ratio is 0.1C (1C=200mAh / g), and the voltage range is 2-4.3V.

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