Wide-temperature range lithium-ion battery electrolyte containing bifunctional lithium salt and preparation method thereof

Abstract

The present invention belongs to the field of lithium-ion battery electrolytes, and provides a wide-temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt and a preparation method thereof. The present invention mixes functionalized lithium salt A, functionalized lithium salt B, boric acid-porphyrin derivatives, phosphorus-based condensed ring imidazoles and other lithium salts with an organic solvent to prepare an electrolyte, wherein the boric acid-porphyrin derivatives capture positive electrode transition metal ions through boric acid groups, and synergize with the oxidation products of phosphorus-based condensed ring imidazoles to form a boron-rich phosphorus-rich composite passivation film on the positive electrode surface, inhibiting solvent oxidation and side reactions; the lactic acid group in the functionalized lithium salt A enhances the lithium ion migration ability at low temperatures; the functionalized lithium salt B utilizes its unsaturated double bond and boric acid group to provide thermal stability under high temperature conditions, and improves structural stability through free radical polymerization. In addition, the decomposition products of these functionalized molecules and lithium salts jointly form a dense boron-rich SEI film on the negative electrode surface, improving interface stability.

Description

Technical Field

[0001] The present invention belongs to the field of lithium-ion battery electrolytes, and relates to a wide-temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt and a preparation method thereof. Background Art

[0002] With the rapid development of electronic devices, renewable energy storage systems, and electric vehicles, the demand for high-performance lithium-ion batteries continues to increase. Lithium-ion batteries have become the most widely used secondary batteries due to their high energy density, long cycle life, and high-efficiency charge and discharge performance. However, the performance of traditional lithium-ion battery electrolytes is poor in a wide temperature range. In the electrolyte system of lithium-ion batteries, lithium salts are one of the core components. Common lithium salts such as lithium hexafluorophosphate perform well at room temperature, but they are easily decomposed in high-temperature environments to produce byproducts such as HF, which seriously affects the stability of the battery's positive electrode materials and electrolytes.

[0003] Chinese patent application with publication number CN107171023A discloses a low-temperature electrolyte for lithium-ion batteries. The electrolyte adopts fluorinated and chlorinated organic carbonate solvents, and utilizes chlorinated propylene carbonate with a larger size as one of the electrolyte solvents to reduce the intercalation reaction of the battery negative electrode graphite, lower the melting point of the electrolyte, and improve the low-temperature characteristics of the electrolyte. However, the electrolyte is difficult to meet the stability of the electrolyte under high temperature conditions; Chinese patent application with publication number CN106252715A discloses a high-temperature electrolyte for lithium-ion batteries, including an organic solvent, a lithium salt mixture and an additive. The high-temperature electrolyte improves the dehydration and deacidification capabilities by adding dehydration and deacidification additives, thereby reducing the reaction of the composite positive electrode material with water and CO2 under high temperature conditions. However, the dehydration and deacidification capabilities of the dehydration and deacidification additives are limited, and the cost of the electrolyte will increase. Summary of the Invention

[0004] In response to the shortcomings of the existing technology, the purpose of the present invention is to provide a wide-temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt and a preparation method thereof. The performance of the electrolyte under low-temperature and high-temperature conditions is improved through the molecular design and synergistic effect of functionalized lithium salt A, functionalized lithium salt B, boric acid-porphyrin derivatives and phosphorus-based condensed ring imidazole. Functionalized lithium salt A optimizes the lithium ion migration ability at low temperatures through lactic acid groups, reduces electrolyte viscosity, and improves low-temperature ionic conductivity; functionalized lithium salt B uses maleic anhydride and boric acid groups to capture free radicals at high temperatures and form a cross-linked structure, enhancing thermal stability; boric acid-porphyrin derivatives capture positive electrode metal ions through boric acid groups and form a boron-rich passivation film at high voltage, inhibiting interfacial side reactions; phosphorus-based condensed ring imidazole generates a phosphorus-rich passivation film at high voltage through phosphorus oxygen groups, improving the stability of the positive electrode interface, thereby meeting the needs of actual production.

[0005] To achieve this object, the present invention adopts the following technical solutions:

[0006] In a first aspect, the present invention provides a method for preparing a wide temperature range lithium ion battery electrolyte containing a bifunctional lithium salt, the preparation method comprising:

[0007] A1, dispersing tetra-p-tolylporphyrin in anhydrous dichloromethane, stirring evenly, then adding 2-chloroacryloyl chloride and triethylamine, stirring and reacting, washing and purifying after completion of the reaction to obtain a chloroacryloylporphyrin derivative, then adding the chloroacryloylporphyrin derivative, biboronic acid pinacol ester, potassium acetate, and tetrakistriphenylphosphine palladium to anhydrous toluene, heating to a first temperature under an argon atmosphere and continuing the reaction, washing and purifying after completion of the reaction to obtain a boric acid-porphyrin derivative;

[0008] A2, dispersing an ortho-diamine in anhydrous dimethylacetamide, adding benzaldehyde under an argon atmosphere, raising the temperature to a second temperature and stirring for reaction, cooling to room temperature after the reaction, adding iodine, raising the temperature to the first temperature and continuing the reaction, filtering and washing to obtain a condensed-ring imidazole intermediate, dispersing the condensed-ring imidazole intermediate in anhydrous tetrahydrofuran, then adding phosphorus oxychloride and triethylamine in sequence, stirring at room temperature, and then raising the temperature to a third temperature for sufficient reaction, washing and rotary evaporation after the reaction is completed to obtain a phosphorus-based condensed-ring imidazole;

[0009] S1, adding boric acid and L-lactic acid to anhydrous ethanol, heating to a third temperature for sufficient reaction, and rotary distilling after the reaction to obtain intermediate A, adding intermediate A and lithium carbonate to anhydrous dimethyl sulfoxide, heating to the first temperature under nitrogen purge and continuing the reaction, adding anhydrous acetone to the reaction solution after the reaction, stirring, collecting the precipitate, washing, filtering, and vacuum drying to obtain functionalized lithium salt A;

[0010] S2, under a nitrogen atmosphere, dispersing maleic anhydride in anhydrous ethanol, adding boric acid and triethylamine, heating to a fourth temperature and reflux reaction to obtain intermediate B, dispersing intermediate B, lithium hydroxide and benzoyl peroxide in anhydrous dimethylacetamide, heating to a first temperature and continuing the reaction, adding anhydrous acetone to the reaction solution after the reaction is completed, collecting the precipitate, washing, filtering and vacuum drying to obtain functionalized lithium salt B;

[0011] S3, adding lithium salt, functionalized lithium salt A, functionalized lithium salt B, boric acid-porphyrin derivative, phosphorus-based condensed ring imidazole and methyl perfluorobutyl ether to an organic solvent, and mixing them evenly to obtain a wide temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt.

[0012] Tetra-p-tolylporphyrin is a highly symmetrical π-conjugated macrocyclic compound consisting of four pyrrole units connected by methylene bridges to form a porphyrin ring. The nitrogen atom in the porphyrin ring has a lone pair of electrons, enabling it to act as a Lewis base and react with electrophilic reagents. Furthermore, the peripheral p-toluene group provides an electron donation effect to the porphyrin, increasing the electron density on the porphyrin ring. The 2-chloroacryloyl chloride molecule contains two reactive sites: the acyl chloride (strong electrophilic) and the allyl chloride (weak electrophilic). The nitrogen atom on the porphyrin ring has strong nucleophilicity and can attack the carbon atom in the acyl chloride group (the carbon on C=O is the electrophilic center). Since the electron cloud of tetra-p-tolylporphyrin is evenly distributed, its peripheral sites, such as the peripheral p-tolyl group, may also react with the acyl chloride through the amino group or hydroxyl group. The carbon atom in the acyl chloride has high electrophilicity due to the strong electron-withdrawing effect of chlorine. The nucleophilic site of the porphyrin attacks the carbon atom of the acyl chloride through the lone pair of electrons, forming a transition state. At the same time, the chloride ion leaves to form chloroacryloylporphyrin and releases hydrogen chloride. Triethylamine combines with hydrogen chloride to form triethylamine salt, thereby maintaining the acid-base balance of the system and preventing degradation of the porphyrin ring. This step introduces a functional group containing a chloroacryloyl group on the porphyrin ring, providing an active site for subsequent coupling reactions. Chloroacryloylporphyrin derivatives serve as organic halide substrates for coupling reactions. The C-Cl bond provides an active site that can undergo cross-coupling with boronic acid reagents. The Pd(0) center in tetrakistriphenylphosphine palladium activates the C-Cl bond in chloroacryloylporphyrin by coordination. Under the action of potassium acetate, the boronic acid group of biphenylboronic acid pinacol ester is activated to form a borate intermediate. The boron group in the borate intermediate undergoes a migration insertion reaction with the Pd-C bond on the porphyrin to form a coupling product. During this process, the boron atom in the boronic acid group is oxidized to borate, and the Pd(II) center releases the final product, the boronic acid-porphyrin derivative, through a reduction elimination reaction and introduces a boronic acid group.

[0013] Porphyrin molecules have a ring structure, and the electron cloud of this ring structure can move freely in the entire porphyrin ring. The presence of the π-conjugated system gives the porphyrin molecule a lower electron migration energy barrier, thereby exhibiting excellent electron transfer ability. In lithium-ion batteries, porphyrin molecules can reduce the resistance to electron transfer in the electrolyte due to their π-conjugated structure. At the same time, the stability and antioxidant properties of porphyrin can reduce side reactions in the electron transfer process under high voltage or high temperature environments, thereby protecting the chemical stability of the electrolyte. In the electrolyte, the migration of lithium ions usually takes place in the form of "solvated lithium ions". Due to the high positive charge density of lithium ions, they will strongly interact with the surrounding solvent molecules (such as carbonate molecules) to form a solvation shell. The solvation shell will significantly increase the migration energy barrier of lithium ions, because lithium ions need to overcome the binding force of solvent molecules during migration to reach the electrode surface to complete the electrochemical reaction. Boric acid group is a typical Lewis acid. The empty orbital on the boron atom can coordinate with the negatively charged oxygen atom (such as the oxygen of the solvent molecule). In the electrolyte, the boric acid group can interact with the lithium ion. The boric acid groups form a weak coordination effect with the lithium ions, replacing some solvent molecules and reducing the desolvation energy barrier that lithium ions need to overcome during migration. The boric acid groups can partially replace the solvent molecules by coordinating with the lithium ions. The weak coordination effect of the boric acid groups not only makes lithium ions easier to desolvate, but also reduces the interaction resistance between lithium ions and solvent molecules during migration, thereby improving the overall migration efficiency of lithium ions. This is particularly evident under wide temperature conditions. At low temperatures, the viscosity of the solvent molecules increases and the desolvation energy barrier is higher. The role of the boric acid groups can reduce the desolvation energy barrier, thereby ensuring the low-temperature performance of the electrolyte.

[0014] The boronic acid groups and porphyrin ring systems contained in the molecules of boric acid-porphyrin derivatives can form a cross-linked structure to generate a polymer film through chemical reaction or physical action. The boronic acid groups can form cross-linked structures with other boronic acid groups or hydroxyl groups through intermolecular dehydration condensation reactions to generate boron oxides, which help to form a dense protective film on the electrode surface under negative electrode potential and high temperature. The rigidity and high molecular weight characteristics of the porphyrin macrocyclic system enable it to provide structural support in the polymer film, thereby improving the mechanical strength and stability of the film. The formed polymer film can prevent solvent molecules or harmful impurities from directly contacting the negative electrode surface, reduce interfacial side reactions, provide stable lithium ion channels, help optimize the transmission efficiency of lithium ions, and reduce ion migration impedance. Electrolytes often contain trace amounts of acidic impurities (such as HF), which can corrode the anode surface and destabilize the SEI film. Boric acid groups, a typical Lewis acid, react with acidic substances like HF to form stable boron fluoride, effectively removing these harmful impurities. Furthermore, during the initial charge and discharge of lithium-ion batteries, solvent decomposition can produce free radicals, which can further lead to side reactions and damage the integrity of the SEI film. Porphyrin macrocyclic rings have excellent electron acceptor capabilities, and their π-conjugated systems can capture free radicals, preventing their diffusion and chain reactions. Boric acid groups can also chemically react with free radicals to remove active species. Boric acid-porphyrin derivatives can form a dense SEI film rich in organic-inorganic components on the anode surface through decomposition, crosslinking, or polymerization. Boron oxides and porphyrin decomposition products act as inorganic and organic components in the SEI film, providing high mechanical strength (the inorganic component inhibits film rupture and dissolution) and good flexibility (the organic component adapts to changes in the negative electrode volume, especially the expansion of the silicon negative electrode). They can reduce the migration resistance of lithium ions and improve interfacial stability. Compared with traditional small molecule solvents (such as ethylene carbonate), boric acid-porphyrin derivatives have a larger molecular weight and higher structural complexity. Their high molecular weight allows boric acid-porphyrin derivatives to form a uniform and dense protective film on the electrode surface at low concentrations. Traditional small molecule solvents are easily volatilized or decomposed at high potentials or temperatures, resulting in their functionality being gradually lost early in the battery cycle and unable to provide long-term stable interfacial protection. Therefore, the addition of boric acid-porphyrin derivatives can improve the thermal and chemical stability of the electrolyte.

[0015] An vicinal diamine reacts with benzaldehyde at elevated temperatures to form an imine intermediate. The amino group of the vicinal diamine acts as a nucleophile to attack the carbonyl carbon of benzaldehyde, forming an unstable hemiacetal intermediate. This intermediate then dehydrates to form a C=N double bond, ultimately forming an imine. The remaining amino group in the vicinal diamine molecule further undergoes intramolecular nucleophilic attack on the C=N double bond of the imine intermediate, forming an imidazole compound with a fused ring structure. Addition of elemental iodine can further stabilize and oxidize the fused ring structure. As a mild oxidant, iodine can further oxidize the imine intermediate or the cyclized imidazole structure to form a more stable fused ring compound. Iodine may also promote double bond electron redistribution within the imidazole ring, thereby improving the conjugation and chemical stability of the fused ring structure. Phosphorus oxychloride is a strong electrophile that reacts with the active site of the imidazole ring to introduce a phosphorus group. The nitrogen atom of the fused imidazole, with its lone pair of electrons, acts as a nucleophile to attack the phosphorus atom of the phosphorus oxychloride, resulting in nucleophilic substitution. The introduction of phosphorus groups imparts antioxidant properties to fused-ring imidazole compounds. The oxygen affinity of the phosphorus atoms enables phosphorus-based fused-ring imidazoles to capture oxidizing species in the electrolyte, thereby inhibiting electrolyte decomposition. The fused-ring imidazole structure itself possesses high thermal stability, and the combination with the phosphorus groups further enhances the compound's stability in high-temperature environments. The high polarity of the azole ring and the conjugated nature of the fused-ring structure enable strong interactions with lithium ions or solvent molecules (such as carbonates) in the electrolyte. The presence of nitrogen atoms and phosphorus groups on the fused-ring imidazole significantly increases the compound's polarity, thereby enhancing the electrolyte's ability to dissolve lithium salts. The flexibility and dispersibility of the fused-ring structure facilitate uniform distribution of the compound in the electrolyte, optimizing the lithium ion transport environment. Phosphorus-based fused-ring imidazoles exhibit excellent performance over a wide temperature range. At high temperatures, the antioxidant capacity and chemical stability of phosphorus-based fused-ring imidazoles effectively inhibit electrolyte decomposition. At low temperatures, the strong solvation effect of the fused-ring imidazole structure on lithium salts and solvents reduces the migration resistance of lithium ions, improving the low-temperature performance of the battery.

[0016] Boric acid and L-lactic acid undergo an esterification reaction under heating to form lactic acid borate. The esterification reaction involves a dehydration reaction between the carboxyl or hydroxyl groups of lactic acid and the hydroxyl groups of boric acid, gradually forming borate bonds to produce a lactic acid-modified boron compound. Intermediate A is a lactic acid-modified borate. The carboxyl group in intermediate A undergoes an acid-base neutralization reaction with lithium carbonate to form a lithium salt. The carboxyl group is then replaced by a lithium ion, yielding the lactic acid borate product (functionalized lithium salt A) as a lithium salt. During the reaction, the boron atom, likely through its Lewis acidity, coordinates intramolecularly with the hydroxyl or carboxylate groups of the lithium lactate salt, further stabilizing the product structure and enhancing the chemical and thermal stability of the lithium salt. The boron group in functionalized lithium salt A may preferentially decompose on the anode surface, forming a boron-rich inorganic / organic mixture. This mixture can form a stable solid electrolyte interface (SEI) film on the anode surface, reducing side reactions on the anode surface and providing a good lithium ion transport pathway. Boron oxides possess high mechanical strength and corrosion resistance, making them effective in reducing the damage to the SEI film caused by electrode volume changes in high-expansion materials such as silicon-based anodes. At the same time, the hydroxyl and carboxylate groups in the lactic acid group have high polarity and can interact strongly with polar solvent molecules (such as ethylene carbonate) through intermolecular hydrogen bonding, which helps the dissolution of the functionalized lithium salt A in the solvent, thereby improving the uniformity of the electrolyte and the lithium salt concentration. In addition, the lactic acid group of the functionalized lithium salt A introduces a large steric hindrance, reduces the electrostatic interaction between molecules, and enhances its solubility in the solvent.

[0017] The lactic acid molecule is an α-hydroxy acid containing hydroxyl and carboxyl groups and possessing a certain degree of polarity. Through an esterification reaction with a boronic acid group, the lactic acid moiety is grafted onto the boronic acid, forming a lithium borate with a lactic acid coordination. The hydroxyl groups in the lactic acid are retained in the lithium salt molecule, imparting a certain degree of stickiness, which helps form a well-adhered interfacial film (SEI) on the negative electrode surface. The presence of the lactic acid group imparts a certain degree of flexibility and polarity to the functionalized lithium salt A molecule, facilitating uniform dispersion in the electrolyte and forming a good interface with the electrode. The boronic acid group is a typical Lewis acid center that can capture trace amounts of water or impurities in the electrolyte through coordination, thereby stabilizing the electrolyte environment. Furthermore, the boronic acid group, by coordinating with lithium ions, can regulate the solvation structure of lithium ions, helping to optimize lithium ion transport performance. The presence of the lactic acid group imparts a certain degree of molecular stickiness to the functionalized lithium salt A. This property facilitates uniform distribution of the functionalized lithium salt A on the electrode surface and facilitates chemical reactions with the electrode surface during the initial lithium insertion process, forming a stable solid electrolyte interface (SEI) film. High viscosity can also reduce the loss of electrolyte on the negative electrode surface, maintain the uniformity of the interfacial reaction, and thus reduce local electrochemical instability. Hydroxyl is a highly reactive functional group. During the first charge process, hydroxyl groups are easily oxidized, dehydrogenated, or cross-linked with other molecules to generate complex organic or inorganic products. These products are important components of the SEI film. The presence of hydroxyl groups also helps to capture by-products in the electrolyte, such as generated HF or trace water, thereby inhibiting the further occurrence of harmful side reactions.

[0018] During the initial charge of a lithium-ion battery, when lithium ions intercalate into the negative electrode (such as graphite or silicon), the electrolyte and its additives undergo irreversible decomposition on the negative electrode surface, forming a solid electrolyte interface (SEI) film. Functionalized lithium salt A, due to its lactic acid and boronic acid groups, can decompose or crosslink during this process, forming a SEI film rich in inorganic / organic compounds. The hydroxyl and carboxyl groups in lactic acid readily undergo redox reactions at negative electrode potentials, forming small organic compounds. These organic compounds can deposit on the negative electrode surface and serve as organic components of the SEI film. Hydroxyl groups can also initiate reactions through free radicals, crosslinking with other molecules to form more complex polymers. This crosslinked structure enhances the mechanical strength and stability of the SEI film. Boric acid groups can undergo a dehydration condensation reaction at negative electrode potentials to form boron oxide, an inorganic compound that can serve as an inorganic component in the SEI film. Boron oxide has good electrochemical stability, can improve the antioxidant properties of the SEI film, and reduce the migration resistance of lithium ions; the decomposition products of the lactic acid group (such as polymers or ester molecules) have certain flexibility and elasticity, which helps to buffer the volume changes of the negative electrode during the cycle (especially the silicon negative electrode), thereby extending the cycle life of the battery. The organic component can also reduce the brittleness of the SEI film, reduce the generation of cracks, and avoid the repeated generation of the SEI film and lithium loss. Inorganic boron oxide provides high mechanical strength and chemical stability in the SEI film, which can inhibit the dissolution or shedding of the SEI film. The presence of inorganic components can also effectively reduce the resistance to lithium ion migration and improve the battery's rate performance.

[0019] Maleic anhydride is an anhydride compound with two carbonyl groups and a five-membered ring structure, exhibiting strong electrophilicity. Boric acid reacts with the anhydride carbonyl groups in maleic anhydride through a nucleophilic addition reaction to form a borate ester intermediate. The resulting intermediate B is a boron compound containing a maleic anhydride derivative. The ester group and unsaturated double bond in intermediate B are key reaction sites. In the presence of lithium hydroxide, the ester group is replaced by lithium ions to form functionalized lithium salt B. The high bond energy of the boronic acid group in functionalized lithium salt B resists decomposition at high temperatures, ensuring the thermal stability of the electrolyte. Furthermore, the boronic acid group interacts with the electrode surface, forming a stable interfacial film at high temperatures, reducing electrolyte decomposition and side reactions. The lithium salt group in functionalized lithium salt B participates in the formation of the SEI film, reducing the film's resistance and improving the interfacial transport of lithium ions. Furthermore, the high polarity of functionalized lithium salt B allows it to dissolve well in carbonate solvents at low temperatures, helping to maintain electrolyte fluidity.

[0020] The porphyrin skeleton has a high thermal decomposition temperature and can maintain stability in high-temperature environments. At the same time, the boric acid groups in the boric acid-porphyrin derivatives can form stable coordination compounds with transition metal ions, reducing metal dissolution in the positive electrode material. The phosphorus-based fused-ring imidazole is preferentially oxidized under high voltage conditions to form a phosphorus-rich passivation film (such as Li3PO4), while the boron oxide (such as B2O3) of the boric acid-porphyrin can work together with the phosphorus oxide to form a composite film. The chemical properties of the composite passivation film are more stable and can effectively inhibit solvent decomposition and side reactions under high voltage. When the decomposition products of functionalized lithium salts A and B (such as borates and organic lithium compounds) form a boron-rich SEI film on the negative electrode surface, the boric acid groups of the boric acid-porphyrin also participate in it, further improving the uniformity and mechanical strength of the film. The π-conjugated structure of porphyrin helps to evenly distribute electrons at the interface, thereby slowing down the electrochemical reaction rate on the negative electrode surface and stabilizing the SEI film. The lactic acid groups in functionalized lithium salt A reduce the viscosity of the electrolyte while optimizing the solvation structure of lithium ions (reducing ion pair formation through solvation shells), thereby improving ionic conductivity at low temperatures. The phosphorus-based condensed-ring imidazole provides oxidative stability at high voltages. The synergistic effect of the two enables the electrolyte to exhibit good performance in both low-temperature and high-voltage ranges. The unsaturated double bonds in functionalized lithium salt B form a more structurally stable interfacial film through free radical polymerization, while the decomposition products of the phosphorus-based condensed-ring imidazole (such as phosphates) can further enhance the chemical inertness and conductivity of the film. At low temperatures, the high polarity of the phosphorus-based condensed-ring imidazole helps maintain the ionic conductivity of the electrolyte. At high voltages and temperatures, its decomposition products form a stable passivation film on the positive electrode surface, significantly improving the oxidative and thermal stability of the electrolyte. At the same time, the lactic acid group in the functionalized lithium salt A enhances the solvation degree of lithium ions and reduces the hydrogen bonding between solvent molecules, thereby reducing the viscosity of the electrolyte at low temperatures and increasing the ion migration rate. The boric acid group preferentially decomposes to form boron-rich compounds, forming a dense SEI film stabilizing the interface on the negative electrode surface. The unsaturated double bonds in the functionalized lithium salt B undergo free radical polymerization to form a cross-linked structure, thereby improving the chemical stability at high temperatures. In addition, the boric acid group of the functionalized lithium salt B can capture the free radicals generated during the decomposition of the electrolyte and inhibit side reactions at high temperatures. Functionalized lithium salt A optimizes the transport performance of lithium ions at low temperatures, while functionalized lithium salt B provides thermal stability and anti-decomposition ability at high temperatures. The combination of the two significantly expands the temperature adaptability range of the electrolyte. The decomposition products of functionalized lithium salts A and B act together on the negative electrode surface to form a boron-containing composite SEI film, which improves its mechanical strength and chemical stability.

[0021] As a preferred technical solution of the present invention, in step A1, the mass volume ratio of the tetra-p-tolylporphyrin to anhydrous dichloromethane is 1 g:20 mL.

[0022] In some optional examples, the mass volume ratio of the tetra-p-tolylporphyrin to 2-chloroacryloyl chloride is 1 g:0.5 mL.

[0023] In some optional examples, the mass volume ratio of the tetra-p-tolylporphyrin to triethylamine is 1 g:1 mL.

[0024] In some optional examples, the stirring reaction time is 3-4h, for example, it can be 3.0h, 3.1h, 3.2h, 3.3h, 3.4h, 3.5h, 3.6h, 3.7h, 3.8h, 3.9h or 4.0h, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0025] In some optional examples, the mass ratio of the chloroacryloylporphyrin derivative, pinacol diboronate, potassium acetate and tetrakistriphenylphosphine palladium is 5:1.5:1:0.2.

[0026] In some optional examples, the mass volume ratio of the chloroacryloylporphyrin derivative to anhydrous toluene is 1 g:20 mL.

[0027] In some optional examples, the first temperature is 90-100°C, for example, it can be 90.0°C, 91.0°C, 92.0°C, 93.0°C, 94.0°C, 95.0°C, 96.0°C, 97.0°C, 98.0°C, 99.0°C or 100.0°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0028] In some optional examples, the first temperature reaction duration is 8-10 hours, for example, it can be 8.0 hours, 8.2 hours, 8.4 hours, 8.6 hours, 8.8 hours, 9.0 hours, 9.2 hours, 9.4 hours, 9.6 hours, 9.8 hours or 10.0 hours, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0029] As a preferred technical solution of the present invention, in step A2, the mass volume ratio of the ortho-diamine to anhydrous dimethylacetamide is 1 g:40 mL.

[0030] In some optional examples, the mass ratio of the o-diamine to benzaldehyde is 1:1.

[0031] In some optional examples, the second temperature is 160-180°C, for example, it can be 160.0°C, 162.0°C, 164.0°C, 166.0°C, 168.0°C, 170.0°C, 172.0°C, 174.0°C, 176.0°C, 178.0°C or 180.0°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0032] In some optional examples, the second temperature reaction duration is 1-2 hours, for example, 1.0 hours, 1.1 hours, 1.2 hours, 1.3 hours, 1.4 hours, 1.5 hours, 1.6 hours, 1.7 hours, 1.8 hours, 1.9 hours or 2.0 hours, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0033] In some optional examples, the mass ratio of the ortho-diamine to elemental iodine is 0.5:1.2.

[0034] In some optional examples, the first temperature reaction duration is 1-2 hours, for example, 1.0 hours, 1.1 hours, 1.2 hours, 1.3 hours, 1.4 hours, 1.5 hours, 1.6 hours, 1.7 hours, 1.8 hours, 1.9 hours or 2.0 hours, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0035] In some optional examples, the mass volume ratio of the fused ring imidazole intermediate to anhydrous tetrahydrofuran is 1 g:20 mL.

[0036] In some optional examples, the mass ratio of the fused-ring imidazole intermediate, phosphorus oxychloride and triethylamine is 1:1.4:1.

[0037] In some optional examples, the stirring time at room temperature is 2-3h, for example, it can be 2.0h, 2.1h, 2.2h, 2.3h, 2.4h, 2.5h, 2.6h, 2.7h, 2.8h, 2.9h or 3.0h, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0038] In some optional examples, the third temperature is 65-75°C, for example, it can be 65.0°C, 66.0°C, 67.0°C, 68.0°C, 69.0°C, 70.0°C, 71.0°C, 72.0°C, 73.0°C, 74.0°C or 75.0°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0039] In some optional examples, the time for sufficient reaction at the third temperature is 4-5h, for example, it can be 4.0h, 4.1h, 4.2h, 4.3h, 4.4h, 4.5h, 4.6h, 4.7h, 4.8h, 4.9h or 5.0h, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0040] As a preferred technical solution of the present invention, in step S1, the mass ratio of boric acid to L-lactic acid is 1:5.

[0041] In some optional examples, the mass volume ratio of L-lactic acid to anhydrous ethanol is 3g:1mL.

[0042] In some optional examples, the reaction time at the third temperature is 4-5h, for example, it can be 4.0h, 4.1h, 4.2h, 4.3h, 4.4h, 4.5h, 4.6h, 4.7h, 4.8h, 4.9h or 5.0h, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0043] In some optional examples, the mass ratio of the intermediate A to lithium carbonate is 25:2.

[0044] In some optional examples, the mass volume ratio of the intermediate A to anhydrous dimethyl sulfoxide is 1 g:1 mL.

[0045] In some optional examples, the reaction time at the first temperature is 6-8h, for example, it can be 6.0h, 6.2h, 6.4h, 6.6h, 6.8h, 7.0h, 7.2h, 7.4h, 7.6h, 7.8h or 8.0h, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0046] As a preferred technical solution of the present invention, in step S2, the mass volume ratio of maleic anhydride to anhydrous ethanol is 1 g:4 mL.

[0047] In some optional examples, the mass ratio of maleic anhydride to boric acid is 5:3.

[0048] In some optional examples, the mass volume ratio of maleic anhydride to triethylamine is 5 g:0.1 mL.

[0049] In some optional examples, the fourth temperature is 70-80°C, for example, it can be 70.0°C, 71.0°C, 72.0°C, 73.0°C, 74.0°C, 75.0°C, 76.0°C, 77.0°C, 78.0°C, 79.0°C or 80.0°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0050] In some optional examples, the reflux reaction time is 2-3h, for example, it can be 2.0h, 2.1h, 2.2h, 2.3h, 2.4h, 2.5h, 2.6h, 2.7h, 2.8h, 2.9h or 3.0h, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0051] In some optional examples, the mass ratio of the intermediate B, lithium hydroxide and benzoyl peroxide is 8:0.6:0.1.

[0052] In some optional examples, the mass volume ratio of the intermediate B to anhydrous dimethylacetamide is 8 g:50 mL.

[0053] In some optional examples, the first temperature reaction duration is 4-6 hours, for example, it can be 4.0 hours, 4.2 hours, 4.4 hours, 4.6 hours, 4.8 hours, 5.0 hours, 5.2 hours, 5.4 hours, 5.6 hours, 5.8 hours or 6.0 hours, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0054] As a preferred technical solution of the present invention, in step S3, the lithium salt is one or both of lithium bis(trifluoromethylsulfonyl)imide and lithium bis(oxalatoborate).

[0055] In some optional examples, the organic solvent is a mixture of ethylene carbonate, ethyl methyl carbonate, and propylene carbonate in a mass ratio of 4:3:3.

[0056] In some optional examples, the concentration of the lithium salt is 0.5-1 mol / L, for example, it can be 0.50 mol / L, 0.55 mol / L, 0.60 mol / L, 0.65 mol / L, 0.70 mol / L, 0.75 mol / L, 0.80 mol / L, 0.85 mol / L, 0.90 mol / L, 0.95 mol / L or 1.00 mol / L, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0057] In some optional examples, the mass ratio of the lithium salt, the functionalized lithium salt A, the functionalized lithium salt B, the porphyrin derivative modified with a boronic acid group, and the phosphorus-containing fused-ring imidazole is 10:1:1:0.5:0.5.

[0058] In some optional examples, the volume ratio of the methyl perfluorobutyl ether to the organic solvent is 1:10.

[0059] In some optional examples, when the lithium salt is lithium bis(trifluoromethylsulfonyl)imide and lithium bis(oxalatoborate), the mass ratio of lithium bis(trifluoromethylsulfonyl)imide to lithium bis(oxalatoborate) is (0.5-1):1, for example, it can be 0.5:1, 0.55:1, 0.6:1, 0.65:1, 0.7:1, 0.75:1, 0.8:1, 0.85:1, 0.9:1, 0.95:1 or 1:1, but is not limited to the listed ratios, and other ratios not listed within this range are also applicable.

[0060] In a second aspect, the present invention provides a wide-temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt prepared by the preparation method described in the first aspect.

[0061] Compared with the prior art, the present invention has the following beneficial effects:

[0062] (1) The lactic acid group has strong lithium affinity and flexibility. By combining with the introduction of the boric acid group, the functionalized lithium salt A can form a dynamic solvation structure with lithium ions in the solution, thereby enhancing the dissociation ability of lithium ions in the solvent, reducing the probability of ion pairs and aggregate formation, and effectively improving the lithium ion migration number. At the same time, the flexible structure of the lactic acid group can reduce the hydrogen bond force between the solvents, so that the viscosity of the electrolyte is reduced in a low-temperature environment, thereby improving the ionic conductivity in a low-temperature environment. The decomposition products of the lactic acid group can form a stable boron-rich SEI film on the negative electrode surface, effectively improving the stability of the negative electrode interface.

[0063] (2) The unsaturated double bonds in the functionalized lithium salt B form a cross-linked network structure through free radical polymerization under high temperature conditions, which improves the thermal stability of the electrolyte under high temperature conditions and inhibits free radical chain reactions. At the same time, the boric acid groups can react quickly with free radicals, capture free radicals, and significantly reduce the destructive effect of free radicals on the electrolyte at high temperatures. The boric acid groups can also be deposited in situ to form a boron-rich SEI film, thereby improving the thermal stability of the electrolyte.

[0064] (3) The boric acid group can selectively coordinate with the transition metal ions dissolved in the positive electrode material, capture and passivate them, and prevent them from further participating in side reactions. The π-conjugated porphyrin ring has excellent oxidative stability and is difficult to be oxidized under high voltage conditions, protecting the electrolyte from high voltage decomposition. The boric acid-porphyrin molecules can interact with the decomposition products of phosphorus-based condensed ring imidazole to form a boron-rich-phosphorus-rich composite passivation film in situ on the positive electrode surface, blocking the direct contact between the solvent and electrolyte and the positive electrode active material, thereby reducing interfacial side reactions. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 Flowchart of the preparation method of the wide-temperature range lithium-ion battery electrolyte containing bifunctional lithium salt provided in Examples 1-4 of the present invention. DETAILED DESCRIPTION

[0066] The technical solutions of the present invention are described in detail below in conjunction with specific embodiments and their accompanying drawings. The embodiments described herein are specific embodiments of the present invention and are used to illustrate the concept of the present invention; these descriptions are explanatory and exemplary and should not be understood as limiting the embodiments of the present invention and the scope of protection of the present invention. In addition to the embodiments described herein, those skilled in the art can also adopt other obvious technical solutions based on the contents disclosed in the claims of this application and its specification, including technical solutions that adopt any obvious replacements and modifications to the embodiments described herein.

[0067] The chemical reagents used in the examples and comparative examples of the present invention are all commercially available products and have not been subjected to any further purification treatment.

[0068] Example 1

[0069] This embodiment provides a method for preparing a wide-temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt, the preparation method specifically comprising the following steps:

[0070] A1: Disperse 1 g of tetra-p-tolylporphyrin in 20 mL of anhydrous dichloromethane, stir evenly, then add 0.5 mL of 2-chloroacryloyl chloride and 1 mL of triethylamine, and react with stirring for 3.2 h. After completion of the reaction, wash and purify to obtain a chloroacryloylporphyrin derivative. Then, add 5 g of the chloroacryloylporphyrin derivative, 1.5 g of biboronic acid pinacol ester, 1 g of potassium acetate, and 0.2 g of tetrakistriphenylphosphine palladium to 100 mL of anhydrous toluene. Under an argon atmosphere, heat to 91°C and continue the reaction for 8.2 h. After completion of the reaction, wash and purify to obtain a boric acid-porphyrin derivative.

[0071] A2: Disperse 0.5 g of o-diamine in 20 mL of anhydrous dimethylacetamide. Under an argon atmosphere, add 0.5 g of benzaldehyde, heat to 169 ° C, stir and react for 1.1 h. After the reaction is completed, cool to room temperature, add 1.2 g of iodine, heat to 97 ° C and continue to react for 1.2 h. Filter and wash to obtain a condensed-ring imidazole intermediate. Disperse 1 g of the condensed-ring imidazole intermediate in 20 mL of anhydrous tetrahydrofuran, then add 1.4 g of phosphorus oxychloride and 1 g of triethylamine in sequence. Stir at room temperature for 2.3 h, then heat to 67 ° C and fully react for 4.2 h. After the reaction is completed, wash and evaporate to obtain a phosphorus-containing condensed-ring imidazole.

[0072] S1, 6g of boric acid and 30g of L-lactic acid were added to 10mL of anhydrous ethanol, heated to 74°C for a full reaction of 4.2h, and rotary distilled to obtain intermediate A. 50g of intermediate A and 4g of lithium carbonate were added to 50mL of anhydrous dimethyl sulfoxide, and the temperature was raised to 92°C under nitrogen purge for a continuous reaction of 6.4h. After the reaction, anhydrous acetone was added to the reaction solution, stirred, and the precipitate was collected, washed, filtered, and vacuum dried to obtain functionalized lithium salt A;

[0073] S2, under a nitrogen atmosphere, 5 g of maleic anhydride was dispersed in 20 mL of anhydrous ethanol, 3 g of boric acid and 0.1 mL of triethylamine were added, and the temperature was raised to 72 ° C. and refluxed for 2.1 h to obtain intermediate B. 8 g of intermediate B, 0.6 g of lithium hydroxide and 0.1 g of benzoyl peroxide were dispersed in 50 mL of anhydrous dimethylacetamide, and the temperature was raised to 99 ° C. The first temperature was continued to react for 4.3 h. After the reaction was completed, anhydrous acetone was added to the reaction solution, and the precipitate was collected, washed, filtered, and vacuum dried to obtain functionalized lithium salt B;

[0074] S3, adding lithium salt, functionalized lithium salt A, functionalized lithium salt B, boric acid-porphyrin derivative, and phosphorus-based condensed ring imidazole in a mass ratio of 10:1:1:0.5:0.5 to methyl perfluorobutyl ether and an organic solvent, wherein the volume ratio of methyl perfluorobutyl ether to the organic solvent is 1:10, the lithium salt is lithium bis(trifluoromethylsulfonyl)imide and lithium dioxalatoborate, and the organic solvent is a mixture of ethylene carbonate, ethyl methyl carbonate, and propylene carbonate. The mixture is evenly mixed to obtain a wide-temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt with a lithium salt concentration of 0.5 mol / L.

[0075] Example 2

[0076] This embodiment provides a method for preparing a wide-temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt, the preparation method specifically comprising the following steps:

[0077] A1: Disperse 2 g of tetra-p-tolylporphyrin in 40 mL of anhydrous dichloromethane, stir well, add 1 mL of 2-chloroacryloyl chloride and 2 mL of triethylamine, and react with stirring for 3.8 h. After completion of the reaction, wash and purify to obtain a chloroacryloylporphyrin derivative. Then, add 5 g of the chloroacryloylporphyrin derivative, 1.5 g of biboronic acid pinacol ester, 1 g of potassium acetate, and 0.2 g of tetrakistriphenylphosphine palladium to 100 mL of anhydrous toluene. Under an argon atmosphere, heat to 97°C and continue the reaction for 9.7 h. After completion of the reaction, wash and purify to obtain a boric acid-porphyrin derivative.

[0078] A2: Disperse 1 g of o-diamine in 40 mL of anhydrous dimethylacetamide. Under an argon atmosphere, add 1 g of benzaldehyde, heat to 162 ° C, stir and react for 1.8 h. After the reaction is completed, cool to room temperature, add 2.4 g of iodine, heat to 92 ° C and continue to react for 1.8 h. Filter and wash to obtain a condensed-ring imidazole intermediate. Disperse 2 g of the condensed-ring imidazole intermediate in 40 mL of anhydrous tetrahydrofuran, then add 2.8 g of phosphorus oxychloride and 2 g of triethylamine in sequence. Stir at room temperature for 2.7 h, then heat to 74 ° C and fully react for 4.8 h. After the reaction is completed, wash and evaporate to obtain a phosphorus-containing condensed-ring imidazole.

[0079] S1, 6g of boric acid and 30g of L-lactic acid were added to 10mL of anhydrous ethanol, heated to 67°C and fully reacted for 4.8h. After the reaction, rotary distillation was performed to obtain intermediate A. 50g of intermediate A and 4g of lithium carbonate were added to 50mL of anhydrous dimethyl sulfoxide. Under nitrogen purge, the temperature was raised to 97°C and the reaction was continued for 7.8h. After the reaction, anhydrous acetone was added to the reaction solution, and the precipitate was collected after stirring, washed, filtered, and vacuum dried to obtain functionalized lithium salt A;

[0080] S2, under a nitrogen atmosphere, 5 g of maleic anhydride was dispersed in 20 mL of anhydrous ethanol, 3 g of boric acid and 0.1 mL of triethylamine were added, and the temperature was raised to 78 ° C. and refluxed for 2.7 h to obtain intermediate B. 8 g of intermediate B, 0.6 g of lithium hydroxide and 0.1 g of benzoyl peroxide were dispersed in 50 mL of anhydrous dimethylacetamide, and the temperature was raised to 92 ° C. The first temperature was continued to react for 4.9 h. After the reaction was completed, anhydrous acetone was added to the reaction solution, and the precipitate was collected, washed, filtered, and vacuum dried to obtain functionalized lithium salt B;

[0081] S3, adding lithium salt, functionalized lithium salt A, functionalized lithium salt B, boric acid-porphyrin derivative, and phosphorus-based condensed ring imidazole in a mass ratio of 10:1:1:0.5:0.5 to methyl perfluorobutyl ether and an organic solvent, wherein the volume ratio of methyl perfluorobutyl ether to the organic solvent is 1:10, the lithium salt is lithium bis(trifluoromethylsulfonyl)imide and lithium dioxalatoborate, and the organic solvent is a mixture of ethylene carbonate, ethyl methyl carbonate, and propylene carbonate. The mixture is evenly mixed to obtain a wide-temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt with a lithium salt concentration of 0.7 mol / L.

[0082] Example 3

[0083] This embodiment provides a method for preparing a wide-temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt, the preparation method specifically comprising the following steps:

[0084] A1: Disperse 1.5 g of tetra-p-tolylporphyrin in 30 mL of anhydrous dichloromethane, stir evenly, add 0.75 mL of 2-chloroacryloyl chloride and 1.5 mL of triethylamine, and react with stirring for 3.6 h. After completion of the reaction, wash and purify to obtain a chloroacryloylporphyrin derivative. Then, add 5 g of the chloroacryloylporphyrin derivative, 1.5 g of biboric acid pinacol ester, 1 g of potassium acetate, and 0.2 g of tetrakistriphenylphosphine palladium to 100 mL of anhydrous toluene. Under an argon atmosphere, heat to 94°C and continue the reaction for 9.2 h. After completion of the reaction, wash and purify to obtain a boric acid-porphyrin derivative.

[0085] A2: Disperse 0.5 g of o-diamine in 20 mL of anhydrous dimethylacetamide. Under an argon atmosphere, add 0.5 g of benzaldehyde, heat to 178 ° C, stir and react for 1.6 h. After the reaction is completed, cool to room temperature, add 1.2 g of iodine, heat to 94 ° C and continue to react for 1.4 h. Filter and wash to obtain a condensed-ring imidazole intermediate. Disperse 1 g of the condensed-ring imidazole intermediate in 20 mL of anhydrous tetrahydrofuran, then add 1.4 g of phosphorus oxychloride and 1 g of triethylamine in sequence. Stir at room temperature for 2.4 h, then heat to 70 ° C and fully react for 4.3 h. After the reaction is completed, wash and evaporate to obtain a phosphorus-containing condensed-ring imidazole.

[0086] S1, 6g of boric acid and 30g of L-lactic acid were added to 10mL of anhydrous ethanol, heated to 70°C and fully reacted for 4.6h. After the reaction, rotary distillation was performed to obtain intermediate A. 50g of intermediate A and 4g of lithium carbonate were added to 50mL of anhydrous dimethyl sulfoxide. Under nitrogen purge, the temperature was raised to 94°C and the reaction was continued for 7.3h. After the reaction, anhydrous acetone was added to the reaction solution, and the precipitate was collected after stirring, washed, filtered, and vacuum dried to obtain functionalized lithium salt A;

[0087] S2, under a nitrogen atmosphere, 5 g of maleic anhydride was dispersed in 20 mL of anhydrous ethanol, 3 g of boric acid and 0.1 mL of triethylamine were added, and the temperature was raised to 76 ° C. and refluxed for 2.6 h to obtain intermediate B. 8 g of intermediate B, 0.6 g of lithium hydroxide and 0.1 g of benzoyl peroxide were dispersed in 50 mL of anhydrous dimethylacetamide, and the temperature was raised to 94 ° C. The first temperature was continued to react for 5.7 h. After the reaction was completed, anhydrous acetone was added to the reaction solution, and the precipitate was collected, washed, filtered, and vacuum dried to obtain functionalized lithium salt B;

[0088] S3, adding lithium salt, functionalized lithium salt A, functionalized lithium salt B, boric acid-porphyrin derivative, and phosphorus-based condensed ring imidazole in a mass ratio of 10:1:1:0.5:0.5 to methyl perfluorobutyl ether and an organic solvent, wherein the volume ratio of methyl perfluorobutyl ether to the organic solvent is 1:10, the lithium salt is lithium bis(trifluoromethylsulfonyl)imide and lithium dioxalatoborate, and the organic solvent is a mixture of ethylene carbonate, ethyl methyl carbonate, and propylene carbonate. The mixture is evenly mixed to obtain a wide-temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt with a lithium salt concentration of 0.9 mol / L.

[0089] Example 4

[0090] This embodiment provides a method for preparing a wide-temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt, the preparation method specifically comprising the following steps:

[0091] A1: Disperse 2 g of tetra-p-tolylporphyrin in 40 mL of anhydrous dichloromethane, stir well, add 1 mL of 2-chloroacryloyl chloride and 2 mL of triethylamine, and react with stirring for 3.4 h. After completion of the reaction, wash and purify to obtain a chloroacryloylporphyrin derivative. Then, add 5 g of the chloroacryloylporphyrin derivative, 1.5 g of biboric acid pinacol ester, 1 g of potassium acetate, and 0.2 g of tetrakistriphenylphosphine palladium to 100 mL of anhydrous toluene. Under an argon atmosphere, heat to 93°C and continue the reaction for 8.6 h. After completion of the reaction, wash and purify to obtain a boric acid-porphyrin derivative.

[0092] A2: Disperse 1 g of o-diamine in 40 mL of anhydrous dimethylacetamide. Under an argon atmosphere, add 1 g of benzaldehyde, heat to 171°C, and stir to react for 1.3 h. After the reaction is completed, cool to room temperature, add 2.4 g of elemental iodine, heat to 96°C, and continue to react for 1.6 h. Filter and wash to obtain a condensed-ring imidazole intermediate. Disperse 2 g of the condensed-ring imidazole intermediate in 40 mL of anhydrous tetrahydrofuran, then add 2.8 g of phosphorus oxychloride and 2 g of triethylamine in sequence. Stir at room temperature for 2.9 h, then heat to 65°C and fully react for 4.5 h. After the reaction is completed, wash and evaporate to obtain a phosphorus-containing condensed-ring imidazole.

[0093] S1, 6g of boric acid and 30g of L-lactic acid were added to 10mL of anhydrous ethanol, heated to 65°C for a full reaction of 4.1h, and rotary distilled to obtain intermediate A. 50g of intermediate A and 4g of lithium carbonate were added to 50mL of anhydrous dimethyl sulfoxide, and the temperature was raised to 99°C under nitrogen purge for a continuous reaction of 6.7h. After the reaction, anhydrous acetone was added to the reaction solution, stirred, and the precipitate was collected, washed, filtered, and vacuum dried to obtain functionalized lithium salt A;

[0094] S2, under a nitrogen atmosphere, disperse 5 g of maleic anhydride in 20 mL of anhydrous ethanol, add 3 g of boric acid and 0.1 mL of triethylamine, heat to 73 ° C and reflux for 2.3 h to obtain intermediate B, disperse 8 g of intermediate B, 0.6 g of lithium hydroxide and 0.1 g of benzoyl peroxide in 50 mL of anhydrous dimethylacetamide, heat to 97 ° C and continue to react for 5.1 h. After the reaction is completed, anhydrous acetone is added to the reaction solution, the precipitate is collected, washed, filtered and vacuum dried to obtain functionalized lithium salt B;

[0095] S3, adding lithium salt, functionalized lithium salt A, functionalized lithium salt B, boric acid-porphyrin derivative, and phosphorus-based condensed ring imidazole in a mass ratio of 10:1:1:0.5:0.5 to methyl perfluorobutyl ether and an organic solvent, wherein the volume ratio of methyl perfluorobutyl ether to the organic solvent is 1:10, the lithium salt is lithium bis(trifluoromethylsulfonyl)imide and lithium dioxalatoborate, and the organic solvent is a mixture of ethylene carbonate, ethyl methyl carbonate, and propylene carbonate. The mixture is evenly mixed to obtain a wide-temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt with a lithium salt concentration of 1.0 mol / L.

[0096] Comparative Example 1

[0097] This comparative example provides a method for preparing a wide-temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt. The difference between the comparative example and Example 1 is that the mass of L-lactic acid in S1 is 60 g, which is 30 g more than that in Example 1. The other process parameters and operating conditions are exactly the same as those in Example 1.

[0098] Comparative Example 2

[0099] This comparative example provides a method for preparing a wide-temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt. The difference between it and Example 1 is that the mass of L-lactic acid in S1 is 5 g, which is 25 g less than that in Example 1. The other process parameters and operating conditions are exactly the same as those in Example 1.

[0100] Comparative Example 3

[0101] This comparative example provides a method for preparing a wide-temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt. The difference between it and Example 1 is that the mass of maleic anhydride in S2 is 10 g, which is 5 g more than that in Example 1. The other process parameters and operating conditions are exactly the same as those in Example 1.

[0102] Comparative Example 4

[0103] This comparative example provides a method for preparing a wide-temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt. The difference between it and Example 1 is that the mass of maleic anhydride in S2 is 1 g, which is 4 g less than that in Example 1. The other process parameters and operating conditions are exactly the same as those in Example 1.

[0104] The electrolyte prepared in the present invention is injected into an unpackaged battery, the positive electrode material is lithium nickel cobalt manganese oxide, and the negative electrode material is graphite. 25°C cycle performance test: At 25±2°C, the lithium-ion battery is charged to 4.2V with a constant current and constant voltage of 0.5C, and the cut-off current is 0.05C, and then discharged to 3.0V with a constant current of 0.5C. The capacity retention rate is calculated after 300 cycles of charge / discharge. 60°C storage capacity retention rate: The lithium-ion battery is cycled and discharged at 0.5C at room temperature for 5 times (4.2-3.0V), and the initial capacity of the battery before storage is recorded. The battery is then charged to a fully charged state of 4.2V with a constant current and constant voltage, and stored in a 60°C environment for 7 days. After the storage is completed, the battery is taken out and cooled to room temperature, and discharged to 3.0V at 0.5C to measure the battery's retention capacity. 50℃ Cycle Performance Test: The lithium-ion battery was cycled and discharged 5 times at 0.5C at room temperature (4.2-3.0V), and the initial capacity of the battery before storage was recorded. At 50±2℃, the battery was charged to 4.2V at 0.5C constant current and constant voltage, with a cut-off current of 0.05C, and discharged to 3.0V at 0.5C constant current. After 300 cycles of charge / discharge, the cycle capacity retention rate was calculated. -20℃ Cycle Capacity Retention Rate: The lithium-ion battery was cycled and discharged 5 times at 0.5C at room temperature (4.2-3.0V), and the initial capacity of the battery before storage was recorded. The battery was charged to 4.2V at 0.5C constant current and constant voltage, with a cut-off current of 0.03C, and then placed in a -20℃ low-temperature box for 5 hours, and discharged to 3.0V at 0.3C constant current. After 300 cycles of charge / discharge, the cycle capacity retention rate was calculated. The test results are shown in Table 1.

[0105] Table 1 Test results of wide temperature range lithium ion battery electrolytes containing bifunctional lithium salts in Examples 1-4 and Comparative Examples 1-4

[0106]

[0107] It can be seen from the data in Table 1 that compared with Example 1, the 25°C cycle capacity retention rate, 60°C storage capacity retention rate, 50°C cycle capacity retention rate, and -20°C cycle capacity retention rate of Comparative Example 1 are all lower than those of Example 1; the 25°C cycle capacity retention rate, 60°C storage capacity retention rate, 50°C cycle capacity retention rate, and -20°C cycle capacity retention rate of Comparative Example 2 are all lower than those of Example 1. This is because the amount of L-lactic acid in Comparative Example 1 is excessive, the proportion of lactic acid groups in the functionalized lithium salt A is relatively increased, the polarity of the functionalized lithium salt A is enhanced, and the solvation capacity is improved, but the dissociation ability of lithium ions may be excessively reduced, resulting in a decrease in the ion migration number, and the concentration of the decomposition products of the lactic acid groups is high under high temperature conditions, which will aggravate the instability of the SEI film and accelerate the side reactions at the negative electrode interface, resulting in a decrease in the capacity retention rate. In Comparative Example 2, L-lactic acid is insufficient, the proportion of lactic acid groups in the functionalized lithium salt A is reduced, the solvation ability of lithium ions is weakened, the formation of the SEI film may be uneven or insufficient, and the number of lithium ion migrations in the electrolyte may be reduced. Under high temperature conditions, it is not enough to form a good boron-rich SEI film on the negative electrode surface, the interface stability is reduced, the capacity retention rate is reduced, and under low temperature conditions, the low-temperature ionic conductivity of the electrolyte is reduced, and the low-temperature cycle performance is reduced.

[0108] It can be seen from the data in Table 1 that compared with Example 1, the 25°C cycle capacity retention rate, 60°C storage capacity retention rate, 50°C cycle capacity retention rate, and -20°C cycle capacity retention rate of Comparative Example 3 are all lower than those of Example 1; the 25°C cycle capacity retention rate, 60°C storage capacity retention rate, 50°C cycle capacity retention rate, and -20°C cycle capacity retention rate of Comparative Example 4 are all lower than those of Example 1. This is because in Comparative Example 3, excessive maleic anhydride causes the proportion of unsaturated double bonds in the functionalized lithium salt B to increase, and excessive unsaturated double bonds may cause unstable polymers or by-products to form in the electrolyte. Under high temperature conditions, excessive unsaturated double bonds are prone to polymerization to form high molecular polymers, resulting in increased electrolyte viscosity and increased interfacial impedance, resulting in a decrease in capacity retention rate. In Comparative Example 4, the amount of maleic anhydride used is too small, and the proportion of unsaturated double bonds in the functionalized lithium salt B is insufficient, which weakens its ability to capture free radicals and form a cross-linked network in the electrolyte, resulting in an uneven or insufficient SEI film. In addition, the insufficient unsaturated double bonds make it impossible for the functionalized lithium salt B to effectively capture free radicals, generating more oxidative byproducts, resulting in a decrease in capacity retention.

[0109] The above description is only a specific embodiment 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 those skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for preparing a wide-temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt, characterized in that: The preparation method comprises: S1, adding boric acid and L-lactic acid to anhydrous ethanol to react to obtain intermediate A, adding intermediate A and lithium carbonate to anhydrous dimethyl sulfoxide to react to obtain functionalized lithium salt A; S2, under a nitrogen atmosphere, dispersing maleic anhydride in anhydrous ethanol, adding boric acid and triethylamine to react to obtain intermediate B, dispersing intermediate B, lithium hydroxide, and benzoyl peroxide in anhydrous dimethylacetamide to react to obtain functionalized lithium salt B; S3, adding lithium salt, functionalized lithium salt A, functionalized lithium salt B, boric acid-porphyrin derivative, phosphorus-containing condensed ring imidazole and methyl perfluorobutyl ether to an organic solvent, and mixing them uniformly to obtain a wide temperature range lithium ion battery electrolyte containing a bifunctional lithium salt; The preparation method of the boric acid-porphyrin derivative comprises: dispersing tetra-p-tolylporphyrin in anhydrous dichloromethane, adding 2-chloroacryloyl chloride and triethylamine to react to obtain a chloroacryloylporphyrin derivative, and then adding the chloroacryloylporphyrin derivative, biboric acid pinacol ester, potassium acetate and tetrakistriphenylphosphine palladium to anhydrous toluene to react to obtain the boric acid-porphyrin derivative; The preparation method of the phosphorus-based condensed ring imidazole comprises: dispersing an ortho-diamine in anhydrous dimethylacetamide, adding benzaldehyde for reaction, cooling the temperature to room temperature after the reaction is completed, adding iodine to react to obtain a condensed ring imidazole intermediate, dispersing the condensed ring imidazole intermediate in anhydrous tetrahydrofuran, and then sequentially adding phosphorus oxychloride and triethylamine to react to obtain the phosphorus-based condensed ring imidazole; The mass ratio of the boric acid to L-lactic acid is 1:5; The mass ratio of maleic anhydride to boric acid is 5:

3.

2. The method for preparing a wide temperature range lithium ion battery electrolyte containing a bifunctional lithium salt according to claim 1, wherein: In S1, The mass ratio of the intermediate A to lithium carbonate is 25:

2.

3. The method for preparing a wide temperature range lithium ion battery electrolyte containing a bifunctional lithium salt according to claim 1, wherein: In S2, The mass ratio of the intermediate B, lithium hydroxide and benzoyl peroxide is 8:0.6:0.

1.

4. The method for preparing a wide temperature range lithium ion battery electrolyte containing a bifunctional lithium salt according to claim 1, wherein: In S3, The lithium salt is one or both of lithium bis(trifluoromethylsulfonyl)imide and lithium bis(oxalatoborate); The organic solvent is a mixture of ethylene carbonate, ethyl methyl carbonate and propylene carbonate in a mass ratio of 4:3:3; The concentration of the lithium salt is 0.5-1 mol / L; The mass ratio of the lithium salt, the functionalized lithium salt A, the functionalized lithium salt B, the porphyrin derivative modified with a boronic acid group, and the phosphorus-containing condensed ring imidazole is 10:1:1:0.5:0.

5.

5. The method for preparing a wide temperature range lithium ion battery electrolyte containing a bifunctional lithium salt according to claim 4, characterized in that: When the lithium salt is lithium bis(trifluoromethylsulfonyl)imide and lithium bis(oxalatoborate), the mass ratio of lithium bis(trifluoromethylsulfonyl)imide to lithium bis(oxalatoborate) is (0.5-1):

1.

6. The method for preparing a wide temperature range lithium ion battery electrolyte containing a bifunctional lithium salt according to claim 1, characterized in that: The mass volume ratio of the tetra-p-tolylporphyrin to 2-chloroacryloyl chloride is 1 g:0.5 mL; The mass ratio of the chloroacryloylporphyrin derivative, pinacol diboron, potassium acetate and tetrakistriphenylphosphine palladium is 5:1.5:1:0.

2.

7. The method for preparing a wide temperature range lithium ion battery electrolyte containing a bifunctional lithium salt according to claim 1, wherein: The mass ratio of the o-diamine to benzaldehyde is 1:1; The mass ratio of the fused-ring imidazole intermediate, phosphorus oxychloride and triethylamine is 1:1.4:

1.

8. A wide temperature range lithium ion battery electrolyte containing a bifunctional lithium salt obtained according to the preparation method according to any one of claims 1 to 7.

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