Wide-temperature-range lithium ion battery electrolyte containing difunctional lithium salt and preparation method of wide-temperature-range lithium ion battery electrolyte

Abstract

The invention belongs to the field of lithium ion battery electrolyte, and provides wide-temperature-range lithium ion battery electrolyte containing difunctional lithium salt and a preparation method of the wide-temperature-range lithium ion battery electrolyte. A functionalized lithium salt A, a functionalized lithium salt B, a boric acid-porphyrin derivative, phosphorus-based condensed ring imidazole and other lithium salts are mixed with an organic solvent to prepare the electrolyte, the boric acid-porphyrin derivative captures positive transition metal ions through a boric acid group, and the boric acid-porphyrin derivative and an oxidation product of the phosphorus-based condensed ring imidazole have a synergistic effect, so that the positive transition metal ions and the phosphorus-based condensed ring imidazole can be converted into positive transition metal ions, and the positive transition metal ions can be converted into negative transition metal ions. A boron-rich and phosphorus-rich composite passivation film is formed on the surface of the positive electrode, so that solvent oxidation and side reaction are inhibited; a lactic acid group in the functionalized lithium salt A enhances the lithium ion migration ability at a low temperature; and the functionalized lithium salt B provides thermal stability under a high-temperature condition by using unsaturated double bonds and boric acid groups, and improves the structural stability through free radical polymerization. In addition, the functional molecules and decomposition products of the lithium salt jointly form a compact boron-rich SEI film on the surface of the negative electrode, so that the interface stability is improved.

Description

Technical Field

[0001] The 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, although they perform well at room temperature, are easily decomposed in high temperature environments to generate byproducts such as HF, which seriously affects the stability of the battery's positive electrode materials and electrolytes.

[0003] A Chinese patent application with publication number CN107171023A discloses a low-temperature electrolyte for a lithium-ion battery. 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, reduce 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; a Chinese patent application with publication number CN106252715A discloses a high-temperature electrolyte for a lithium-ion battery, 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 view of the shortcomings of the prior art, 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, and to improve the performance of the electrolyte under low and high temperature conditions 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 temperature through lactic acid groups, reduces the viscosity of the electrolyte, and improves the low temperature ion conductivity; functionalized lithium salt B uses maleic anhydride and boric acid groups to capture free radicals at high temperature and form a cross-linked structure to enhance 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 to inhibit interface side reactions; phosphorus-based condensed ring imidazole generates a phosphorus-rich passivation film at high voltage through phosphorus oxygen groups, improves the stability of the positive electrode interface, and thus meets 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, adding 2-chloroacryloyl chloride and triethylamine, stirring to react, washing and purifying after the reaction is completed to obtain a chloroacryloylporphyrin derivative, then adding the chloroacryloylporphyrin derivative, biboric acid pinacol ester, potassium acetate and tetrakistriphenylphosphine palladium to anhydrous toluene, heating to a first temperature under an argon atmosphere to continue the reaction, washing and purifying after the reaction is completed to obtain a boric acid-porphyrin derivative;

[0008] A2, dispersing an ortho-diamine in anhydrous dimethylacetamide, adding benzaldehyde under an argon atmosphere, heating to a second temperature and stirring for reaction, cooling to room temperature after the reaction, adding iodine, heating to a 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 sequentially adding phosphorus oxychloride and triethylamine, stirring at room temperature and then heating 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, rotary distilling to obtain intermediate A after the reaction is completed, adding intermediate A and lithium carbonate to anhydrous dimethyl sulfoxide, heating to the first temperature under nitrogen purge conditions to continue the reaction, adding anhydrous acetone to the reaction solution after the reaction is completed, collecting the precipitate after stirring, 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 for reflux reaction to obtain an intermediate B, dispersing the intermediate B, lithium hydroxide and benzoyl peroxide in anhydrous dimethylacetamide, heating to a first temperature for continuous reaction, adding anhydrous acetone to the reaction solution after the reaction is completed, collecting the precipitate, washing, filtering and vacuum drying to obtain a functionalized lithium salt B;

[0011] 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 into 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 with a porphyrin ring formed by four pyrrole units connected by a methylene bridge. The nitrogen atom on the porphyrin ring has a lone pair of electrons, which allows it to act as a Lewis base and react with electrophilic reagents. In addition, 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 active sites, acyl chloride (strong electrophilicity) and allyl chloride (weak electrophilicity). 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-tolyl porphyrin is evenly distributed, its peripheral sites, such as the peripheral p-toluene group, may also react with the acyl chloride through the amino group or the 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 porphyrin attacks the carbon atom of the acyl chloride through the lone pair of electrons to form a transition state; at the same time, the chloride ion leaves to generate chloroacryloyl porphyrin and releases hydrogen chloride. Triethylamine combines with hydrogen chloride to generate triethylamine salt, thereby maintaining the acid-base balance of the system and preventing the degradation of the porphyrin ring. This step introduces a functional group containing a chloroacryloyl group on the porphyrin ring, providing an active site for the subsequent coupling reaction. Chloroacryloylporphyrin derivatives are used as organic halide substrates for coupling reactions. The C-Cl bond provides an active site that can undergo cross-coupling with the boronic acid reagent. 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 generate 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. In this process, the boron atom in the boronic acid group is oxidized to borate, and the Pd(II) center releases the final product 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 makes the porphyrin molecule have 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, lithium ions 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. The 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 can partially replace the solvent molecules by coordinating with the lithium ions. The weak coordination effect of the boric acid groups not only makes it easier for lithium ions 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, improve the mechanical strength and stability of the film, and the formed polymer film can block 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. The electrolyte usually contains trace acidic impurities (such as HF), which can corrode the negative electrode surface and cause the instability of the SEI film. Boric acid groups are a typical Lewis acid that reacts with acidic substances such as HF to form stable borofluoride, effectively removing these harmful impurities. In addition, during the first charge and discharge process of lithium-ion batteries, solvent decomposition may produce free radicals, which will further lead to side reactions and destroy the integrity of the SEI film. The porphyrin macrocyclic system has good electron acceptor ability, and its π-conjugated system can capture free radicals to prevent the diffusion and chain reaction of free radicals. Boric acid groups can also remove active species by chemically reacting with free radicals. Boric acid-porphyrin derivatives can form a dense SEI film rich in organic-inorganic components on the negative electrode surface through decomposition, cross-linking or polymerization. Boron oxide and porphyrin decomposition products act as inorganic and organic components in the SEI film, providing high mechanical strength (inorganic components inhibit film rupture and dissolution) and good flexibility (organic components adapt to changes in the volume of the negative electrode, especially the expansion of the silicon negative electrode), which can reduce the migration impedance of lithium ions and improve interface stability. Compared with traditional small molecule solvents (such as ethylene carbonate), boric acid-porphyrin derivatives have a larger molecular weight and higher structural complexity. The large 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 high temperatures, resulting in the gradual loss of their functionality in the early stages of the battery cycle and the inability to provide long-term stable interface protection. Therefore, the addition of boric acid-porphyrin derivatives can improve the thermal and chemical stability of the electrolyte.

[0015] The ortho-diamine reacts with benzaldehyde under high temperature conditions to generate an imine intermediate. The amino group of the ortho-diamine acts as a nucleophilic agent to attack the carbonyl carbon of benzaldehyde to form an unstable hemiacetal intermediate. The intermediate is dehydrated to form a C=N double bond, thereby forming an imine. The remaining amino group in the ortho-diamine molecule further undergoes intramolecular nucleophilic attack with the C=N double bond of the imine intermediate to generate an imidazole compound with a fused ring structure. The addition of iodine can further promote the stability and oxidation of the fused ring structure. As a mild oxidant, iodine can further oxidize the imine intermediate or the cyclized imidazole structure to generate a more stable fused ring compound. Iodine may also promote the redistribution of double bond electrons in the imidazole ring, thereby improving the conjugation and chemical stability of the fused ring structure. Phosphorus oxychloride is a strong electrophilic reagent that can react with the active site in the imidazole ring to complete the introduction of the phosphorus group. The nitrogen atom of the fused ring imidazole has a lone pair of electrons and acts as a nucleophilic agent to attack the phosphorus atom of phosphorus oxychloride in the reaction to undergo nucleophilic substitution. The introduction of phosphorus groups endows polycyclic imidazole compounds with antioxidant ability. The oxygen affinity of phosphorus atoms enables phosphorus-based polycyclic imidazoles to capture oxidizing substances in the electrolyte, thereby inhibiting the decomposition of the electrolyte. The polycyclic imidazole structure itself has high thermal stability, and the combination with phosphorus groups further improves the stability of the compound in high temperature environments. The high polarity of the azole ring and the conjugation of the polycyclic structure enable it to interact strongly with lithium ions or solvent molecules (such as carbonates) in the electrolyte. The presence of nitrogen atoms and phosphorus groups on polycyclic imidazoles significantly increases the polarity of the compound, thereby enhancing the electrolyte's ability to dissolve lithium salts; the flexibility and dispersibility of the polycyclic structure contribute to the uniform distribution of the compound in the electrolyte and optimize the lithium ion transmission environment. Phosphorus-based polycyclic imidazoles exhibit good performance under wide temperature conditions. The antioxidant ability and chemical stability of phosphorus-based polycyclic imidazoles at high temperatures effectively inhibit the decomposition of the electrolyte; the strong solvation effect of the polycyclic imidazole structure on lithium salts and solvents at low temperatures can reduce the migration resistance of lithium ions and improve the low-temperature performance of the battery.

[0016] Boric acid and L-lactic acid undergo an esterification reaction under heating conditions to generate lactic acid borate. The esterification reaction is a dehydration reaction between the carboxyl or hydroxyl group of lactic acid and the hydroxyl group of boric acid, gradually forming a borate ester bond to generate 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 generate a lithium salt. The carboxyl group is replaced by lithium ions to generate a lactic acid borate product (functionalized lithium salt A) in the form of a lithium salt. During the reaction, the boron atom may undergo intramolecular coordination with the hydroxyl or carboxylate of the lithium lactate salt through its Lewis acidity, further stabilizing the product structure and enhancing the chemical stability and thermal stability of the lithium salt. The boron group in the functionalized lithium salt A may preferentially decompose on the negative electrode surface to generate a boron-rich inorganic / organic mixture, which can form a stable solid electrolyte interface film (SEI film) on the negative electrode surface, reduce side reactions on the negative electrode surface and provide a good lithium ion transmission channel. Boron oxide has high mechanical strength and corrosion resistance, and can effectively reduce the damage to the SEI film caused by electrode volume changes in high expansion materials such as silicon-based negative electrodes. At the same time, the hydroxyl group and carboxylate group 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 having a certain polarity. Through the esterification reaction with the boric acid group, the lactic acid part is grafted onto the boric acid to generate a lithium borate with lactic acid coordination. The hydroxyl group in the lactic acid is retained in the lithium salt molecule, which has a certain viscosity and helps to form an interface film (SEI film) with good adhesion on the negative electrode surface. The presence of the lactic acid group makes the functionalized lithium salt A molecule have a certain flexibility and polarity, which is conducive to the uniform dispersion of the molecule in the electrolyte and the formation of a good interface with the electrode. The boric acid group is a typical Lewis acid center, which can capture trace moisture or impurities in the electrolyte through coordination, thereby stabilizing the electrolyte environment. In addition, the boric acid group can regulate the solvation structure of lithium ions by coordinating with lithium ions, which helps to optimize the lithium ion transmission performance. Functionalized lithium salt A has a certain molecular viscosity due to the presence of lactic acid groups. Such characteristics make it easy for functionalized lithium salt A to be evenly distributed on the electrode surface, and it is easier to react chemically with the electrode surface during the first lithium insertion process to form a stable solid electrolyte interface film (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 is easily oxidized, dehydrogenated, or cross-linked with other molecules to generate complex organic or inorganic products, which are important components of the SEI film. The presence of hydroxyl also helps to capture by-products in the electrolyte, such as HF or trace water, thereby inhibiting the further occurrence of harmful side reactions.

[0018] During the first charge of a lithium-ion battery, when lithium ions are embedded in the negative electrode (such as graphite or silicon), the electrolyte and its additives will undergo irreversible decomposition on the surface of the negative electrode to form a solid electrolyte interface film (SEI film). Functionalized lithium salt A can decompose or cross-link in this process due to the lactic acid and boric acid groups in its molecules to form an SEI film rich in inorganic / organic mixtures. The hydroxyl and carboxyl groups in lactic acid are prone to redox reactions at the negative electrode potential to generate small molecular organic compounds. These organic substances can be deposited on the surface of the negative electrode and serve as organic components of the SEI film. Hydroxyl groups may also initiate reactions through free radicals and undergo cross-linking reactions with other molecules to form more complex polymers. This cross-linking structure improves the mechanical strength and stability of the SEI film. Boric acid groups may generate boron oxides through dehydration condensation reactions at negative electrode potentials. This is 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, which has strong electrophilicity. Boric acid reacts with the anhydride carbonyl group in maleic anhydride to form a borate ester intermediate. The generated intermediate B is a boron compound containing maleic anhydride derivatives. The ester group and unsaturated double bond in intermediate B are key reaction sites. Under the action of lithium hydroxide, the ester group is replaced by lithium ions to form functionalized lithium salt B. The boric acid group in functionalized lithium salt B has a high bond energy and can resist decomposition under high temperature conditions, ensuring the thermal stability of the electrolyte. The boric acid group can form a stable interface film at high temperature through interaction with the electrode surface, reducing the occurrence of electrolyte decomposition and side reactions. The lithium salt group in functionalized lithium salt B can participate in the formation process of SEI film, reduce the resistance of the film and improve the interfacial transmission capacity of lithium ions. At the same time, the polarity of functionalized lithium salt B is high, and it can be well dissolved in carbonate solvents under low temperature conditions, which helps to maintain the fluidity of the electrolyte.

[0020] The porphyrin skeleton has a high thermal decomposition temperature and can maintain stability in a high temperature environment. 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 condensed ring imidazole is preferentially oxidized under high voltage conditions to form a phosphorus-rich passivation film (such as Li3PO4), and 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 the 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 are also involved, further improving the uniformity and mechanical strength of the film. The π-conjugated structure of porphyrin helps to evenly distribute electrons on the interface, thereby slowing down the electrochemical reaction rate on the negative electrode surface and stabilizing the SEI film. The lactic acid groups in the functionalized lithium salt A reduce the viscosity of the electrolyte and optimize the solvation structure of lithium ions (reducing the formation of ion pairs through the solvation shell effect), thereby improving the 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 the functionalized lithium salt B form a more structurally stable interface film through free radical polymerization, and the decomposition products of the phosphorus-based condensed ring imidazole (such as phosphate) 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 voltage and high temperature, its decomposition products form a stable passivation film on the positive electrode surface, significantly improving the oxidative stability and thermal stability of the electrolyte. At the same time, the lactic acid group in the functionalized lithium salt A reduces the viscosity of the electrolyte at low temperatures and increases the ion migration rate by enhancing the solvation degree of lithium ions and reducing the hydrogen bonding between solvent molecules. The boric acid group preferentially decomposes to generate 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. 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. The functionalized lithium salt A optimizes the transport performance of lithium ions at low temperatures, while the 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 the functionalized lithium salts A and B act together on the negative electrode surface to form a boron-containing composite SEI film, thereby improving 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 tetra-p-tolylporphyrin to 2-chloroacryloyl chloride is 1 g:0.5 mL.

[0023] In some optional examples, the mass volume ratio of 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, bipyraclostrobin, 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 instances, 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 h, for example, 8.0 h, 8.2 h, 8.4 h, 8.6 h, 8.8 h, 9.0 h, 9.2 h, 9.4 h, 9.6 h, 9.8 h or 10.0 h, 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 instances, the second temperature is 160-180°C, for example, it may 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 1-2h, for example, 1.0h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, 1.9h or 2.0h, 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 1-2h, for example, 1.0h, 1.1h, 1.2h, 1.3h, 1.4h, 1.5h, 1.6h, 1.7h, 1.8h, 1.9h or 2.0h, 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 instances, 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 the 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 instances, 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, 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 condensed-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 dioxalate borate, the mass ratio of lithium bis(trifluoromethylsulfonyl)imide and lithium dioxalate borate 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 the 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 aggregates, and effectively increasing the number of lithium ion migration. At the same time, the flexible structure of the lactic acid group can reduce the hydrogen bonding force between 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 and capture free radicals, significantly reducing the destructive effect of free radicals on the electrolyte under high temperature. 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 the 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 the electrolyte and the positive electrode active material, thereby reducing interfacial side reactions. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 A flow chart of a method for preparing a wide temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt provided in Examples 1-4 of the present invention. DETAILED DESCRIPTION

[0066] The technical solution of the present invention is described in detail below in conjunction with specific embodiments and their accompanying drawings. The embodiments recorded herein are specific embodiments of the present invention, which are used to illustrate the concept of the present invention; these descriptions are explanatory and exemplary and should not be construed as limitations on the embodiments of the present invention and the scope of protection of the present invention. In addition to the embodiments recorded 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 recorded herein.

[0067] The chemical reagents used in the examples and comparative examples of the present invention are all commercially available products without 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, and the preparation method specifically comprises the following steps:

[0070] A1, disperse 1g of tetra-p-tolylporphyrin in 20mL of anhydrous dichloromethane, stir evenly, add 0.5mL of 2-chloroacryloyl chloride and 1mL of triethylamine, stir and react for 3.2h, wash and purify after the reaction is completed to obtain a chloroacryloylporphyrin derivative, then add 5g of chloroacryloylporphyrin derivative, 1.5g of biboric acid pinacol ester, 1g of potassium acetate and 0.2g of tetrakistriphenylphosphine palladium to 100mL of anhydrous toluene, heat to 91°C under argon atmosphere and continue to react for 8.2h, wash and purify after the reaction is completed to obtain a boric acid-porphyrin derivative;

[0071] A2, 0.5g of o-diamine was dispersed in 20mL of anhydrous dimethylacetamide, 0.5g of benzaldehyde was added under argon atmosphere, the temperature was raised to 169°C and stirred for reaction for 1.1h, after the reaction was completed, the temperature was lowered to room temperature, 1.2g of iodine was added, the temperature was raised to 97°C and the reaction was continued for 1.2h, and the condensed ring imidazole intermediate was obtained by filtration and washing, 1g of the condensed ring imidazole intermediate was dispersed in 20mL of anhydrous tetrahydrofuran, and 1.4g of phosphorus oxychloride and 1g of triethylamine were added in sequence, the temperature was raised to 67°C and the reaction was fully reacted for 4.2h after stirring at room temperature for 2.3h, after the reaction was completed, the temperature was washed and rotary evaporated to obtain the phosphorus-based condensed ring imidazole;

[0072] S1, add 6g of boric acid and 30g of L-lactic acid to 10mL of anhydrous ethanol, heat to 74°C to fully react for 4.2h, and perform rotary distillation to obtain intermediate A after the reaction is completed. Add 50g of intermediate A and 4g of lithium carbonate to 50mL of anhydrous dimethyl sulfoxide, and heat to 92°C to continue the reaction for 6.4h under nitrogen purge. After the reaction is completed, add anhydrous acetone to the reaction solution, stir and collect the precipitate, wash and filter, and then vacuum dry to obtain functionalized lithium salt A;

[0073] S2, under nitrogen atmosphere, 5g of maleic anhydride was dispersed in 20mL of anhydrous ethanol, 3g of boric acid and 0.1mL of triethylamine were added, the temperature was raised to 72°C and refluxed for 2.1h to obtain intermediate B, 8g of intermediate B, 0.6g of lithium hydroxide and 0.1g of benzoyl peroxide were dispersed in 50mL of anhydrous dimethylacetamide, the temperature was raised to 99°C and the first temperature was continued for 4.3h, after the reaction was completed, anhydrous acetone was added to the reaction solution, the precipitate was collected, washed and filtered, and then 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-containing 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 having 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, and the preparation method specifically comprises the following steps:

[0077] A1, disperse 2g of tetra-p-tolylporphyrin in 40mL of anhydrous dichloromethane, stir evenly, add 1mL of 2-chloroacryloyl chloride and 2mL of triethylamine, stir and react for 3.8h, wash and purify after the reaction is completed to obtain a chloroacryloylporphyrin derivative, then add 5g of chloroacryloylporphyrin derivative, 1.5g of biboric acid pinacol ester, 1g of potassium acetate and 0.2g of tetrakistriphenylphosphine palladium to 100mL of anhydrous toluene, heat to 97°C under argon atmosphere and continue to react for 9.7h, wash and purify after the reaction is completed to obtain a boric acid-porphyrin derivative;

[0078] A2, disperse 1g of ortho-diamine in 40mL of anhydrous dimethylacetamide, add 1g of benzaldehyde under argon atmosphere, heat to 162℃ and stir to react for 1.8h, cool to room temperature after the reaction, add 2.4g of iodine, heat to 92℃ and continue to react for 1.8h, filter and wash to obtain a polycyclic imidazole intermediate, disperse 2g of polycyclic imidazole intermediate in 40mL of anhydrous tetrahydrofuran, add 2.8g of phosphorus oxychloride and 2g of triethylamine in sequence, stir at room temperature for 2.7h, heat to 74℃ and fully react for 4.8h, wash and evaporate after the reaction to obtain a phosphorus-based polycyclic imidazole;

[0079] S1, add 6g of boric acid and 30g of L-lactic acid to 10mL of anhydrous ethanol, heat to 67°C to fully react for 4.8h, and perform rotary distillation to obtain intermediate A after the reaction is completed. Add 50g of intermediate A and 4g of lithium carbonate to 50mL of anhydrous dimethyl sulfoxide, and heat to 97°C under nitrogen purge to continue the reaction for 7.8h. After the reaction is completed, add anhydrous acetone to the reaction solution, stir and collect the precipitate, wash and filter, and then vacuum dry to obtain functionalized lithium salt A;

[0080] S2, under nitrogen atmosphere, 5g of maleic anhydride was dispersed in 20mL of anhydrous ethanol, 3g of boric acid and 0.1mL of triethylamine were added, the temperature was raised to 78°C and refluxed for 2.7h to obtain intermediate B, 8g of intermediate B, 0.6g of lithium hydroxide and 0.1g of benzoyl peroxide were dispersed in 50mL of anhydrous dimethylacetamide, the temperature was raised to 92°C and the first temperature was continued for 4.9h, after the reaction was completed, anhydrous acetone was added to the reaction solution, the precipitate was collected, washed and filtered, and then 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-containing 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 having 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, and the preparation method specifically comprises the following steps:

[0084] A1, disperse 1.5g of tetra-p-tolylporphyrin in 30mL of anhydrous dichloromethane, stir evenly, add 0.75mL of 2-chloroacryloyl chloride and 1.5mL of triethylamine, stir and react for 3.6h, wash and purify after the reaction is completed to obtain a chloroacryloylporphyrin derivative, then add 5g of chloroacryloylporphyrin derivative, 1.5g of biboric acid pinacol ester, 1g of potassium acetate and 0.2g of tetrakistriphenylphosphine palladium to 100mL of anhydrous toluene, heat to 94°C under argon atmosphere and continue to react for 9.2h, wash and purify after the reaction is completed to obtain a boric acid-porphyrin derivative;

[0085] A2, 0.5g of o-diamine was dispersed in 20mL of anhydrous dimethylacetamide, 0.5g of benzaldehyde was added under argon atmosphere, the temperature was raised to 178°C and stirred for reaction for 1.6h, after the reaction, the temperature was lowered to room temperature, 1.2g of iodine was added, the temperature was raised to 94°C and the reaction was continued for 1.4h, filtered and washed to obtain a polycyclic imidazole intermediate, 1g of the polycyclic imidazole intermediate was dispersed in 20mL of anhydrous tetrahydrofuran, 1.4g of phosphorus oxychloride and 1g of triethylamine were added in sequence, the temperature was raised to 70°C and fully reacted for 4.3h after stirring at room temperature for 2.4h, after the reaction was completed, the temperature was washed and rotary evaporated to obtain a phosphorus-based polycyclic imidazole;

[0086] S1, add 6g of boric acid and 30g of L-lactic acid to 10mL of anhydrous ethanol, heat to 70°C to fully react for 4.6h, and then perform rotary distillation to obtain intermediate A after the reaction is completed. Add 50g of intermediate A and 4g of lithium carbonate to 50mL of anhydrous dimethyl sulfoxide, and heat to 94°C to continue the reaction for 7.3h under nitrogen purge. After the reaction is completed, add anhydrous acetone to the reaction solution, stir and collect the precipitate, wash and filter, and then vacuum dry to obtain functionalized lithium salt A;

[0087] S2, under nitrogen atmosphere, 5g of maleic anhydride was dispersed in 20mL of anhydrous ethanol, 3g of boric acid and 0.1mL of triethylamine were added, the temperature was raised to 76°C and refluxed for 2.6h to obtain intermediate B, 8g of intermediate B, 0.6g of lithium hydroxide and 0.1g of benzoyl peroxide were dispersed in 50mL of anhydrous dimethylacetamide, the temperature was raised to 94°C and the first temperature was continued for 5.7h, after the reaction was completed, anhydrous acetone was added to the reaction solution, the precipitate was collected, washed and filtered, and then 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-containing 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 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, and the preparation method specifically comprises the following steps:

[0091] A1, disperse 2g of tetra-p-tolylporphyrin in 40mL of anhydrous dichloromethane, stir evenly, add 1mL of 2-chloroacryloyl chloride and 2mL of triethylamine, stir and react for 3.4h, wash and purify after the reaction is completed to obtain a chloroacryloylporphyrin derivative, then add 5g of chloroacryloylporphyrin derivative, 1.5g of biboric acid pinacol ester, 1g of potassium acetate and 0.2g of tetrakistriphenylphosphine palladium to 100mL of anhydrous toluene, heat to 93°C under argon atmosphere and continue to react for 8.6h, wash and purify after the reaction is completed to obtain a boric acid-porphyrin derivative;

[0092] A2, disperse 1g of ortho-diamine in 40mL of anhydrous dimethylacetamide, add 1g of benzaldehyde under argon atmosphere, heat to 171℃, stir and react for 1.3h, cool to room temperature after the reaction, add 2.4g of iodine, heat to 96℃ and continue to react for 1.6h, filter and wash to obtain a polycyclic imidazole intermediate, disperse 2g of polycyclic imidazole intermediate in 40mL of anhydrous tetrahydrofuran, add 2.8g of phosphorus oxychloride and 2g of triethylamine in sequence, stir at room temperature for 2.9h, heat to 65℃ and fully react for 4.5h, wash and evaporate after the reaction to obtain a phosphorus-based polycyclic imidazole;

[0093] S1, add 6g of boric acid and 30g of L-lactic acid to 10mL of anhydrous ethanol, heat to 65°C to fully react for 4.1h, and rotary distill to obtain intermediate A after the reaction is completed. Add 50g of intermediate A and 4g of lithium carbonate to 50mL of anhydrous dimethyl sulfoxide, and heat to 99°C under nitrogen purge to continue the reaction for 6.7h. After the reaction is completed, add anhydrous acetone to the reaction solution, stir and collect the precipitate, wash and filter, and vacuum dry to obtain functionalized lithium salt A;

[0094] S2, under nitrogen atmosphere, 5g of maleic anhydride was dispersed in 20mL of anhydrous ethanol, 3g of boric acid and 0.1mL of triethylamine were added, the temperature was raised to 73°C and refluxed for 2.3h to obtain intermediate B, 8g of intermediate B, 0.6g of lithium hydroxide and 0.1g of benzoyl peroxide were dispersed in 50mL of anhydrous dimethylacetamide, the temperature was raised to 97°C and the first temperature was continued for 5.1h, after the reaction was completed, anhydrous acetone was added to the reaction solution, the precipitate was collected, washed and filtered, and then 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-containing 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 having 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 it and Example 1 is that the mass of L-lactic acid in S1 is 60g, which is 30g more than that in Example 1, and 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, and 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 10g, which is 5g more than that in Example 1, and 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 1g, which is 4g less than that in Example 1, and 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℃ cycle performance test: At 25±2℃, the lithium-ion battery is charged to 4.2V with 0.5C constant current and constant voltage, and the cut-off current is 0.05C, and then discharged to 3.0V with 0.5C constant current. The capacity retention rate is calculated after 300 cycles of charge / discharge. 60℃ 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. Then the battery is charged to 4.2V fully charged with constant current and constant voltage, and stored in a 60℃ environment for 7 days. After the storage, 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 is charged and discharged 5 times (4.2-3.0V) at 0.5C at room temperature, and the initial capacity of the battery before storage is recorded. At 50±2℃, the battery is charged to 4.2V with 0.5C constant current and constant voltage, the cut-off current is 0.05C, and the battery is discharged to 3.0V with 0.5C constant current. After 300 cycles of charge / discharge, the cycle capacity retention rate is calculated. -20℃ cycle capacity retention rate: The lithium-ion battery is charged and discharged 5 times (4.2-3.0V) at 0.5C at room temperature, and the initial capacity of the battery before storage is recorded. The battery is charged to 4.2V with 0.5C current constant current and constant voltage, the cut-off current is 0.03C, and then the battery is placed in a low temperature box at -20℃ for 5h, and discharged to 3.0V with 0.3C current constant current, and the cycle capacity retention rate is calculated after 300 cycles of charge / discharge. The test results are shown in Table 1.

[0105] Table 1 Test results of wide temperature range lithium ion battery electrolyte 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 there is an excess of L-lactic acid in Comparative Example 1, 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 negative electrode interface side reactions, 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, and the capacity retention rate is reduced. 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 will lead to an increase in the proportion of unsaturated double bonds in the functionalized lithium salt B, and too many unsaturated double bonds may lead to the formation of unstable polymers or by-products in the electrolyte. Excessive unsaturated double bonds are prone to polymerization under high temperature conditions to generate 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, resulting in the generation of more oxidative by-products, resulting in a decrease in the capacity retention rate.

[0109] The above description is only a specific implementation mode 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 are 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 into an organic solvent, and mixing them evenly to obtain a wide temperature range lithium-ion battery electrolyte containing a bifunctional lithium salt.

2. 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 preparation method of the boric acid-porphyrin derivative comprises: A1, tetra-p-tolylporphyrin is dispersed in anhydrous dichloromethane, 2-chloroacryloyl chloride and triethylamine are added to react to obtain a chloroacryloylporphyrin derivative, and then the chloroacryloylporphyrin derivative, bipyraclostrobin, potassium acetate and tetrakistriphenylphosphine palladium are added to anhydrous toluene to react to obtain a boric acid-porphyrin derivative.

3. 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 preparation method of the phosphorus-based condensed ring imidazole comprises: A2, dispersing the ortho-diamine in anhydrous dimethylacetamide, adding benzaldehyde to react, cooling to room temperature after the reaction, adding iodine to react, and obtaining a condensed ring imidazole intermediate, dispersing the condensed ring imidazole intermediate in anhydrous tetrahydrofuran, and then adding phosphorus oxychloride and triethylamine in sequence to react to obtain a phosphorus-based condensed ring imidazole.

4. The method for preparing a wide temperature range lithium ion battery electrolyte containing a bifunctional lithium salt according to claim 1, characterized in that: In S1, The mass ratio of the boric acid to L-lactic acid is 1:5; The mass ratio of the intermediate A to lithium carbonate is 25:

2.

5. The method for preparing a wide temperature range lithium ion battery electrolyte containing a bifunctional lithium salt according to claim 1, characterized in that: In S2, The mass ratio of maleic anhydride to boric acid is 5:3; The mass ratio of the intermediate B, lithium hydroxide and benzoyl peroxide is 8:0.6:0.

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: In S3, The lithium salt is one or two 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 boric acid group, and the phosphorus-containing condensed ring imidazole is 10:1:1:0.5:0.

5.

7. The method for preparing a wide temperature range lithium ion battery electrolyte containing a bifunctional lithium salt according to claim 6, characterized in that: When the lithium salt is lithium bis(trifluoromethylsulfonyl)imide and lithium bis(oxalate borate), the mass ratio of lithium bis(trifluoromethylsulfonyl)imide to lithium bis(oxalate borate) is (0.5-1):

1.

8. The method for preparing a wide temperature range lithium ion battery electrolyte containing a bifunctional lithium salt according to claim 2, characterized in that: In A1, 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, bipyraclostrobin, potassium acetate and tetrakistriphenylphosphine palladium is 5:1.5:1:0.

2.

9. The method for preparing a wide temperature range lithium ion battery electrolyte containing a bifunctional lithium salt according to claim 3, characterized in that: In A2, 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.

10. 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 9.

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