High-entropy electrolyte, preparation method thereof and lithium ion battery

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a high-entropy electrolyte, a preparation method thereof and a lithium ion battery. The invention provides a high-entropy electrolyte. The high-entropy electrolyte comprises a lithium salt, an organic solvent and an additive, the types of the organic solvents are more than or equal to 6, and the mass of each organic solvent accounts for more than 6% of the total mass of the organic solvents; the types of the lithium salts are greater than or equal to 2, and the mass of each lithium salt accounts for more than 20% of the total mass of the lithium salts. According to the high-entropy electrolyte disclosed by the invention, the low-temperature performance of the electrolyte is improved, and meanwhile, other performances such as normal-temperature cycle performance and high-temperature cycle performance can be considered by improving the confusion degree of the organic solvent and the lithium salt, increasing the entropy value of the electrolyte and adjusting the solvation structure of lithium ions; the high-entropy electrolyte is used in the lithium ion battery, so that the lithium ion battery has excellent low-temperature DCR, low-temperature discharge performance, normal-temperature cycle performance and high-temperature cycle performance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a high entropy electrolyte, a preparation method thereof, and a lithium ion battery. Background Art

[0002] At present, electric vehicles have rapidly occupied the automobile market with advantages such as low energy consumption and zero carbon emissions, and are favored by the majority of users. With the development of technology, problems such as short driving range and long charging time have gradually been solved. However, low-temperature performance is a pain point that restricts the application of electric vehicles in cold areas. Improving the charging and discharging capacity of batteries at low temperatures has also become the focus of technical research in the industry.

[0003] In lithium-ion batteries, the electrolyte has a significant impact on the low-temperature performance of the battery. The electrolyte is composed of solvents, lithium salts and additives. In order for the battery to have good low-temperature performance, the electrolyte needs to have the following properties: high lithium ion conductivity at low temperatures, good lithium ion conductivity of the SEI membrane, and rapid charge transfer between the electrolyte and the electrode surface.

[0004] In the existing industry technology, low-temperature electrolytes are basically designed by combining low-melting-point solvents, high-dissociation lithium salts and low-impedance film-forming additives. Low-melting-point solvents are carboxylates such as EA and EP, lithium salts are often LiFSI as high-dissociation lithium salts, and film-forming additives are boron-containing additives such as LiBF4 and other additives containing S and P elements.

[0005] Carboxylic acid esters have better low-temperature performance, but poor electrode compatibility, especially poor reduction stability on the negative electrode surface. Due to the large amount of carboxylic acid esters used and the single solvent structure, the electrolyte stability is low, and its conductivity is limited by the amount of carboxylic acid esters used, it is difficult to achieve a balance between low-temperature performance and other properties. At this time, in order to take into account other room temperature and high temperature performance, the industry often increases the amount of film-forming additives to improve high-temperature performance, resulting in complex formulations and increased costs.

[0006] In view of this, this invention is proposed. Summary of the invention

[0007] The first purpose of the present invention is to provide a high entropy electrolyte, which improves the low-temperature capability while taking into account other performance and cost requirements by increasing the disorder of organic solvents and lithium salts, increasing the entropy value of the electrolyte, and adjusting the solvation structure of lithium ions.

[0008] The second object of the present invention is to provide a method for preparing a high entropy electrolyte with simple steps.

[0009] A third object of the present invention is to provide a lithium ion battery having excellent low temperature DCR, low temperature discharge performance, room temperature cycle performance and high temperature cycle performance.

[0010] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are particularly adopted:

[0011] In a first aspect, the present invention provides a high entropy electrolyte, comprising a lithium salt, an organic solvent and an additive;

[0012] The number of types of organic solvents is ≥6, and the mass of each organic solvent accounts for more than 6% of the total mass of organic solvents; the number of types of lithium salts is ≥2, and the mass of each lithium salt accounts for more than 20% of the total mass of lithium salts.

[0013] Further, the organic solvent includes cyclic carbonate, linear carbonate and carboxylic acid ester;

[0014] The number of the cyclic carbonates is ≥2, the number of the linear carbonates is ≥2, and the number of the carboxylates is ≥2.

[0015] Furthermore, the mass of the cyclic carbonate accounts for less than 30% of the total mass of the organic solvent.

[0016] Furthermore, in the electrolyte, the total content of the lithium salt is 10wt% to 20wt%.

[0017] Furthermore, the lithium salt includes lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide.

[0018] Furthermore, the additives include vinyl sulfate and vinylene carbonate.

[0019] Furthermore, in the electrolyte, the content of the additive is 0.5wt% to 1.5wt%.

[0020] In a second aspect, the present invention also provides a method for preparing the high entropy electrolyte as described above, comprising the following steps:

[0021] After the lithium salt, the organic solvent and the additive are uniformly mixed, the high entropy electrolyte is obtained.

[0022] In a third aspect, the present invention further provides a lithium-ion battery comprising the high entropy electrolyte as described above.

[0023] Furthermore, the lithium-ion battery also includes a positive electrode material and a negative electrode material;

[0024] The positive electrode material comprises a nickel-cobalt-manganese ternary positive electrode material, wherein the mass percentage of nickel in the nickel-cobalt-manganese ternary positive electrode material is ≥ 60%;

[0025] The negative electrode material includes graphite and / or silicon compounds.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] The high entropy electrolyte of the present invention can improve the low-temperature capability of the electrolyte while taking into account other performance and cost requirements by increasing the disorder of the organic solvent and the lithium salt, increasing the entropy value of the electrolyte, and adjusting the solvation structure of lithium ions; the high entropy electrolyte is used in lithium-ion batteries to improve the low-temperature DCR and low-temperature discharge performance of the lithium-ion batteries, while taking into account excellent normal temperature cycle performance and high temperature cycle performance. DETAILED DESCRIPTION

[0028] The technical scheme of the present invention will be clearly and completely described below in conjunction with specific embodiments, but it will be appreciated by those skilled in the art that the following described embodiments are part of embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention, and should not be considered as limiting the scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work, all belong to the scope of protection of the present invention. If specific conditions are not indicated in the embodiments, they are carried out according to normal conditions or conditions recommended by the manufacturer. If the manufacturer is not indicated in the reagents or instruments used, they are all conventional products that can be purchased commercially.

[0029] A high entropy electrolyte, a preparation method thereof, and a lithium-ion battery according to an embodiment of the present invention are described in detail below.

[0030] In some embodiments of the present invention, a high entropy electrolyte is provided, comprising a lithium salt, an organic solvent and an additive;

[0031] The number of types of organic solvents is ≥6, and the mass of each organic solvent accounts for more than 6% of the total mass of organic solvents; the number of types of lithium salts is ≥2, and the mass of each lithium salt accounts for more than 20% of the total mass of lithium salts.

[0032] The high entropy electrolyte of the present invention improves the disorder of the electrolyte solvent and the lithium salt, increases the entropy value of the electrolyte, and adjusts the solvation structure of lithium ions, thereby improving the low-temperature capability while taking into account the normal temperature and high temperature cycle performance.

[0033] In order to achieve good discharge capacity of lithium-ion batteries at low temperatures, the electrolyte needs to take into account both mass transfer and charge transfer capabilities. Mass transfer requires that the electrolyte has sufficiently good lithium ion transport capacity at low temperatures, and charge transfer requires that lithium ions can quickly transfer charge on the electrode surface. In order to improve the above capabilities, the electrolyte is required to have the highest possible conductivity and the fastest possible desolvation ability. Based on the theory of lithium ion transport in electrolytes, the movement of lithium ions includes two pathways: transitions between lithium ion solvent molecules and direct transmission of solvated lithium ions. Therefore, accelerating the transition of lithium ions and the movement of solvated lithium ions can improve the lithium ion transport capacity. Based on the concept of entropy, the greater the disorder of the system, the less constraint it has on lithium ions, which can improve lithium ion conductivity.

[0034] Based on the above principle, the high entropy electrolyte of the present invention increases the chaos and disorder of the electrolyte components by increasing the types of solvents and lithium salts (increasing the molecular diversity of the electrolyte), thereby improving the low-temperature transmission ability and desolvation ability of lithium ions. According to the solvation theory of ions, lithium ions and solvents form a one-to-many solvation structure. In the traditional electrolyte with EC as the main solvent, the solvation sheath of lithium ions contains 4 EC molecules coordinated with it, and other linear esters are distributed on the periphery. This coordination structure leads to a large volume of lithium ion solvation clusters, so the lithium ions move slowly. In the high entropy electrolyte of the present invention, the molecules have sufficient diversity, and other cyclic carbonates and linear esters with similar polarity to EC can also participate in the coordination structure of lithium ions, break the original solvation sheath restriction, reduce the size of the solvation clusters, and thus improve the migration ability of lithium ions; the molecules in the high entropy electrolyte have a high degree of chaos and disorder, which can reduce the constraint of the solvent on lithium ions and enhance the transmission of lithium ions in the liquid phase; at the interface between the positive and negative electrodes, lithium ions undergo desolvation and are embedded in the electrode. For the process of lithium ions embedding into the graphite negative electrode (charging), the high entropy electrolyte solvent has a high degree of disorder, and the constraint on lithium ions is smaller, which can achieve a more efficient desolvation of lithium ions, thereby achieving a matching of the negative electrode with stronger fast charging capability; therefore, the high entropy electrolyte of the present invention can improve the electrolyte kinetics at low temperatures and improve the low temperature performance of the battery. In addition, since the high entropy electrolyte of the present invention does not need to rely on the use of a large number of single carboxylates, the electrolyte stability is enhanced, and other properties can also be taken into account.

[0035] In the high entropy electrolyte of the present invention, the number of types of organic solvents has a significant effect on the low-temperature discharge performance of the battery. When the number of types of organic solvents is not less than 6, the low-temperature performance can be effectively maintained, and the high entropy electrolyte formed by multiple organic solvents has a favorable effect on the low-temperature performance; the number of types of lithium salts also affects the performance of the electrolyte. When the number of types of lithium salts is less than two, the low-temperature performance of the electrolyte will also deteriorate.

[0036] In some embodiments of the present invention, the organic solvent includes cyclic carbonates, linear carbonates, and carboxylates;

[0037] The number of cyclic carbonates is ≥2, the number of linear carbonates is ≥2, and the number of carboxylic acid esters is ≥2.

[0038] In the organic solvent of the electrolyte of the present invention, the diversity of cyclic carbonate types, the diversity of linear carbonate types and the diversity of carboxylic acid ester types all have an impact on the low-temperature performance; when the types of cyclic carbonates, linear carbonates and carboxylic acid esters are less than 2, the low-temperature DCR will increase significantly and the low-temperature energy retention rate will decrease.

[0039] In some embodiments of the present invention, the mass of the cyclic carbonate is less than 30% of the total mass of the organic solvent; preferably, the mass of the cyclic carbonate is 12% to 30% of the total mass of the organic solvent; typically but not limiting, for example, the mass of the cyclic carbonate is 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, 30% of the total mass of the organic solvent or a range consisting of any two of them.

[0040] When the amount of cyclic carbonate is high or low, the low-temperature performance of the electrolyte decreases, resulting in deterioration of the low-temperature discharge capability of the battery. This is related to the dielectric constant and viscosity of the cyclic carbonate. Its high dielectric constant ensures the dissociation degree of the lithium salt, but the high viscosity is not conducive to ion conduction.

[0041] In some embodiments of the present invention, the cyclic carbonate includes at least two of ethylene carbonate (EC), propylene carbonate (PC) and fluoroethylene carbonate.

[0042] In some embodiments of the present invention, the linear carbonate includes at least two of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and propylene carbonate.

[0043] In some embodiments of the present invention, the carboxylic acid ester includes at least two of methyl formate, methyl acetate, ethyl formate, ethyl acetate (EA), methyl propionate, and ethyl propionate (EP).

[0044] In some embodiments of the present invention, the organic solvent further includes an aprotic organic solvent.

[0045] In some embodiments of the present invention, the total content of lithium salt in the electrolyte is 10wt% to 20wt%; typically but not limiting, for example, the total content of lithium salt in the electrolyte can be 10%, 12%, 14%, 16%, 18%, 20% or a range of any two thereof.

[0046] In the electrolyte of the present invention, the total content of lithium salt is within the above range, which can take into account the effects of ionic strength and viscosity on low-temperature performance; when the total content of lithium salt is higher or lower, the low-temperature performance of the electrolyte is reduced.

[0047] In some embodiments of the present invention, the lithium salt includes lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI).

[0048] In some embodiments of the present invention, the additive includes diethyl sulfate (DTD) and diethyl sulfide (VC).

[0049] In some embodiments of the present invention, the content of the additive in the electrolyte is 0.5wt% to 1.5wt%; typically but not limitatively, for example, the content of the additive in the electrolyte can be 0.5wt%, 0.8wt%, 1wt%, 1.3wt%, 1.5wt% or a range of any two thereof.

[0050] In the electrolyte of the present invention, the additive plays an important role in the impedance and battery cycle performance. Too high an amount will lead to a large impedance battery and affect the low-temperature performance; too low an amount will lead to insufficient electrode protection and deterioration of the cycle and high-temperature performance.

[0051] In some embodiments of the present invention, a method for preparing the high entropy electrolyte is also provided, comprising the following steps:

[0052] After the lithium salt, the organic solvent and the additive are evenly mixed, a high entropy electrolyte is obtained.

[0053] In some embodiments of the present invention, a lithium-ion battery is provided, comprising the high-entropy electrolyte.

[0054] The high entropy electrolyte of the present invention is used in lithium-ion batteries to improve the low-temperature DCR and low-temperature discharge performance of the lithium-ion batteries, while taking into account other normal-temperature and high-temperature cycle performances.

[0055] In some embodiments of the present invention, the lithium-ion battery further comprises a positive electrode material and a negative electrode material;

[0056] The positive electrode material includes a nickel-cobalt-manganese ternary positive electrode material, and the mass percentage of nickel in the nickel-cobalt-manganese ternary positive electrode material is ≥ 60%;

[0057] The negative electrode material includes graphite and / or silicon compounds.

[0058] Examples 1 to 11

[0059] The method for preparing the high entropy electrolyte provided in this embodiment comprises the following steps:

[0060] In an argon atmosphere glove box with a water content of <10ppm, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethyl propionate (EP), and ethyl acetate (EA) are mixed to form an organic solvent; then LiPF6, LiFSI and additives are added and mixed evenly to obtain an electrolyte; wherein the composition of the organic solvent and the mass ratio of each component, the composition of the lithium salt in the electrolyte and the mass percentage of each component, and the composition of the additive in the electrolyte and the mass percentage of each component are shown in Table 1.

[0061] Table 1

[0062]

[0063]

[0064] Comparative Examples 1 to 9

[0065] The preparation method of the high entropy electrolyte provided in this comparative example comprises the following steps:

[0066] In an argon atmosphere glove box with a water content of <10ppm, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethyl propionate (EP), and ethyl acetate (EA) are mixed to form an organic solvent; then LiPF6, LiFSI and additives are added and mixed evenly to obtain an electrolyte; wherein the composition of the organic solvent and the mass ratio of each component, the composition of the lithium salt in the electrolyte and the mass percentage of each component, and the composition of the additive in the electrolyte and the mass percentage of each component are shown in Table 2.

[0067] Table 2

[0068]

[0069]

[0070] Test example

[0071] Lithium ion batteries were assembled using the high entropy electrolytes of Examples 1 to 11 and Comparative Examples 1 to 9, respectively. The electrochemical performance of each battery was tested. The results are shown in Table 3.

[0072] A method for preparing a lithium-ion battery comprises the following steps:

[0073] The positive electrode active material (ternary NCM, LiNi 0.6 Co 0.2 Mn 0.2 O2), polyvinylidene fluoride as a binder and Super P as a conductive agent are mixed in a weight ratio of 98:1:1, N-methylpyrrolidone (NMP) is added, and the mixture is stirred under the action of a vacuum mixer until the system becomes uniform and transparent to obtain a positive electrode slurry; the positive electrode slurry is evenly coated on an aluminum foil; the aluminum foil is dried at room temperature and then transferred to an oven for drying, and then cold pressed and cut to obtain a positive electrode sheet;

[0074] Artificial graphite is used as the negative electrode active material, Super P is used as the conductive agent, sodium carboxymethyl cellulose (CMC-Na) is used as the thickener, and styrene-butadiene rubber (SBR) is used as the binder, and the mixture is mixed in a mass ratio of 96:1:1:2, and deionized water is added to obtain the negative electrode slurry under the action of a vacuum mixer; the negative electrode slurry is evenly coated on the negative electrode current collector copper foil; the copper foil is dried at room temperature and then transferred to an oven for drying, and then cold pressed and cut to obtain the negative electrode sheet;

[0075] A polypropylene film (PP) with a thickness of 12 μm was used as an isolation film, and the positive electrode sheet, isolation film, and negative electrode sheet prepared above were stacked in sequence, so that the isolation film was located between the positive and negative electrodes to play an isolating role; then they were wrapped with an aluminum-plastic film, transferred to a vacuum oven at 120°C for drying, and sealed after injecting 3.0 g / Ah of electrolyte, and the electrolyte was formed to finally prepare a soft-pack battery (i.e., a lithium-ion battery) with a capacity of 1 Ah.

[0076] The electrochemical performance test method is as follows:

[0077] Secondary battery cycle test: In an oven at a specified temperature (room temperature 25°C or high temperature 45°C), cyclic charge and discharge are performed at a current of 1C within a specified potential range, and the discharge capacity of each cycle is recorded. The test is terminated when the battery capacity reaches 80% of the first cycle capacity.

[0078] Secondary battery DC resistance (DCR) test: At a specified temperature, when the battery is discharged to 50% SOC (state of charge, reflecting the remaining capacity of the battery) at a current of 1C, the current is increased to 4C and maintained for 30s. The difference between the updated stable voltage and the original platform voltage is detected. The ratio of its value to the 4C current value is the DC resistance of the battery. The DCR test result after the battery is fully charged for the first time is the initial DCR of the battery.

[0079] Secondary battery low temperature discharge capacity retention rate test: After the secondary battery is fully charged, it is placed in a -20°C constant temperature box. After sufficient cooling, it is discharged at a 1C rate to the cut-off voltage, and its capacity is compared with the percentage of the initial discharge capacity.

[0080] The battery's charge and discharge cut-off voltage is: 2.8~4.4V.

[0081] Table 3

[0082]

[0083]

[0084] As can be seen from Table 3, when the number of organic solvents is less than 6 and the number of cyclic carbonates is less than 2, the low-temperature DCR of the battery increases significantly and the low-temperature energy retention rate decreases, indicating that the diversity of the cyclic carbonates has an effect on the low-temperature performance; from Example 1, Example 3 and Comparative Example 2, when the number of linear carbonates is less than 2, the low-temperature DCR of the battery increases significantly and the low-temperature energy retention rate decreases, indicating that the diversity of the linear carbonates has an effect on the low-temperature performance; from Example 1, Example 4 and Comparative Example 3, when the number of carboxylic acid esters is less than 2, the low-temperature DCR of the battery increases significantly and the low-temperature energy retention rate decreases, indicating that the diversity of the carboxylic acid esters has an effect on the low-temperature performance. Based on the above analysis, it can be seen that the type of organic solvent has a significant effect on the low-temperature discharge performance of the battery. Only when the number of solvents is not less than 6, the low-temperature performance can be effectively maintained, which also proves that the high-entropy solvent structure formed by a sufficient number of solvent types will have a favorable effect on the low-temperature performance.

[0085] Comparison of Example 1, Example 2, Comparative Example 4 and Comparative Example 5 shows that when the amount of cyclic carbonate exceeds 30% or is less than 12%, the low-temperature discharge capability of the battery will deteriorate. This is related to the dielectric constant and viscosity of the cyclic carbonate. Its high dielectric constant ensures the dissociation degree of the lithium salt, but the high viscosity is not conducive to ion conduction.

[0086] Comparison of Examples 6 to 9 and Comparative Examples 6 to 7 shows that when the number of lithium salt types is less than two, the low temperature performance of the battery will also deteriorate; when the total lithium salt content is less than 10% or greater than 20%, the low temperature performance of the battery will also deteriorate. A reasonable range of lithium salt concentration can take into account the effects of ionic strength and viscosity on low temperature performance.

[0087] Comparison of Example 1, Example 10, Example 11, Comparative Example 8 and Comparative Example 9 shows that additives play an important role in impedance and battery cycle performance. Too high a dosage will lead to a large impedance of the impedance battery and affect the low-temperature performance; too low a dosage will lead to insufficient electrode protection and deterioration of the cycle and high-temperature performance; when the dosage of the additive is higher than 1.5wt%, the low-temperature performance of the battery is significantly deteriorated; when the dosage is lower than 0.5wt%, the cycle performance of the battery is seriously deteriorated.

[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A high entropy electrolyte, characterized in that: Including lithium salts, organic solvents and additives; The number of types of organic solvents is ≥6, and the mass of each organic solvent accounts for more than 6% of the total mass of organic solvents; the number of types of lithium salts is ≥2, and the mass of each lithium salt accounts for more than 20% of the total mass of lithium salts.

2. The high entropy electrolyte according to claim 1, characterized in that The organic solvent includes cyclic carbonate, linear carbonate and carboxylic acid ester; The number of the cyclic carbonates is ≥2, the number of the linear carbonates is ≥2, and the number of the carboxylates is ≥2.

3. The high entropy electrolyte according to claim 2, characterized in that The mass of the cyclic carbonate accounts for less than 30% of the total mass of the organic solvent.

4. The high entropy electrolyte according to claim 3, characterized in that In the electrolyte, the total content of the lithium salt is 10wt% to 20wt%.

5. The high entropy electrolyte according to claim 4, characterized in that The lithium salts include lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide.

6. The high entropy electrolyte according to claim 4, characterized in that The additives include vinyl sulfate and vinylene carbonate.

7. The high entropy electrolyte according to claim 6, characterized in that In the electrolyte, the content of the additive is 0.5wt% to 1.5wt%.

8. The method for preparing a high entropy electrolyte according to any one of claims 1 to 7, characterized in that: The steps include: After the lithium salt, the organic solvent and the additive are uniformly mixed, the high entropy electrolyte is obtained.

9. A lithium ion battery, characterized in that: The invention comprises the high entropy electrolyte as described in any one of claims 1 to 7.

10. The lithium ion battery according to claim 9, characterized in that: The lithium-ion battery also includes a positive electrode material and a negative electrode material; The positive electrode material comprises a nickel-cobalt-manganese ternary positive electrode material, wherein the mass percentage of nickel in the nickel-cobalt-manganese ternary positive electrode material is ≥ 60%; The negative electrode material includes graphite and / or silicon compounds.

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