CN119518095A

CN119518095A - Electrolyte and lithium-ion battery

Info

Publication number: CN119518095A
Application number: CN202411644071.5A
Authority: CN
Inventors: 余乐; 夏赫一
Original assignee: Envision Power Technology Jiangsu Co Ltd; Envision Ruitai Power Technology Shanghai Co Ltd
Current assignee: Yuanjing Power Technology Ordos Co ltd; Envision Power Technology Jiangsu Co Ltd; Envision Ruitai Power Technology Shanghai Co Ltd
Priority date: 2024-11-15
Filing date: 2024-11-15
Publication date: 2025-02-25

Abstract

本发明涉及电解液技术领域，具体为一种电解液及锂离子电池。电解液包括：有机溶剂、锂盐、稀释剂和助溶剂。有机溶剂包括碳酸酯类溶剂，锂盐溶于有机溶剂，稀释剂不参与锂离子在有机溶剂中的溶剂化反应，并且具有阻燃性；助溶剂为芳香族化合物，助溶剂的极性介于稀释剂和有机溶剂之间。碳酸酯类溶剂、稀释剂和助溶剂在电解液中的质量分数分别为w₁、w₂和w₃，且30％≤w₁≤65％、15％≤w₂≤50％、5％≤w₃≤30％。本发明中的电解液可兼顾锂离子电池的快充性能和热安全性能。The present invention relates to the technical field of electrolytes, and specifically to an electrolyte and a lithium-ion battery. The electrolyte comprises: an organic solvent, a lithium salt, a diluent and a cosolvent. The organic solvent comprises a carbonate solvent, the lithium salt is dissolved in the organic solvent, the diluent does not participate in the solvation reaction of lithium ions in the organic solvent, and has flame retardancy; the cosolvent is an aromatic compound, and the polarity of the cosolvent is between that of the diluent and the organic solvent. The mass fractions of the carbonate solvent, the diluent and the cosolvent in the electrolyte are _w1 , _w2 and _w3 , respectively, and 30% _≤w1≤65 %, 15% _≤w2≤50 %, and 5% _≤w3≤30 %. The electrolyte in the present invention can take into account both the fast charging performance and the thermal safety performance of the lithium-ion battery.

Description

Electrolyte and lithium ion battery

Technical Field

The invention relates to the technical field of electrolyte, in particular to electrolyte and a lithium ion battery.

Background

The lithium ion battery has the advantages of high specific energy, high battery voltage, wide working temperature range, long storage life and the like, is one of the most popular charging technologies at present, and is widely applied to the fields of electric automobiles, aerospace, portable equipment and the like.

Along with the continuous expansion of application scenes, the lithium ion battery is required to work normally in normal temperature and high temperature environments, and also needs to have good quick charge capacity, so that an electrolyte with good quick charge capacity is required. In order to improve the battery's fast charge capacity, the electrolyte needs to maintain high ionic conductivity and low interfacial resistance, so a low viscosity linear carbonate solvent is typically added, while the additive minimizes or avoids the use of high resistance positive and negative film forming additives to avoid increasing the resistance to lithium transport at the electrode/electrolyte interface. However, such a fast-charge electrolyte would again have a problem of poor thermal safety.

Disclosure of Invention

Aiming at the problems, the invention provides the electrolyte and the lithium ion battery, and the electrolyte and the lithium ion battery can improve the quick charge performance of the lithium ion battery and simultaneously consider the thermal safety performance by adding the proper low-viscosity cosolvent into the electrolyte in a solvent system using a thermally stable diluent.

In one aspect, the invention provides an electrolyte comprising an organic solvent, a lithium salt, a diluent, and a co-solvent. The organic solvent comprises a carbonic ester solvent, the mass fraction of the carbonic ester solvent in the electrolyte is w ₁,30％≤w₁ -65%, the lithium salt is dissolved in the organic solvent, the diluent does not participate in the solvation reaction of lithium ions in the organic solvent and has flame retardance, the mass fraction of the diluent in the electrolyte is w ₂,15％≤w₂ -50%, the cosolvent is an aromatic compound, the polarity of the cosolvent is between the diluent and the organic solvent, and the mass fraction of the cosolvent in the electrolyte is w ₃,5％≤w₃ -30%.

Optionally, the mass ratio of the cosolvent to the diluent is n, n=w ₃/w₂, and n satisfies the following relationship that n is 1/4.ltoreq.n.ltoreq.2.

Alternatively, the diluent is a C3-C8 perfluoro or polyfluoro substituted ether diluent.

Optionally, the diluent is any one or a combination of at least two of ethyl nonafluorobutyl ether, perfluorobutyl methyl ether and perfluorobutyl ethyl ether, and the cosolvent is any one or a combination of at least two of o-difluorobenzene, m-difluorobenzene, p-difluorobenzene, 1,3, 5-trifluorobenzene, 1,2, 3-trifluorobenzene, 1,2, 4-trifluorobenzene, toluene, ethylbenzene and trifluoromethylbenzene.

Optionally, the carbonate solvent is composed of a cyclic carbonate solvent and a linear carbonate solvent, and the mass fraction of the cyclic carbonate solvent in the carbonate solvent is 5% -20%.

Optionally, the cyclic carbonate solvent is ethylene carbonate and/or propylene carbonate.

Optionally, the lithium salt comprises any one or a combination of at least two of lithium bis (fluorosulfonyl) imide, lithium bis (trifluoromethylsulfonyl) imide, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate and lithium hexafluoroarsenate, and the mass fraction of the lithium salt in the electrolyte is 10% -20%.

Optionally, the electrolyte comprises an additive, the additive comprises a cyclic carbonate additive and a lithium salt additive, wherein the cyclic carbonate additive comprises any one or a combination of at least two of ethylene carbonate, fluoroethylene carbonate and ethylene carbonate, the lithium salt additive comprises any one or a combination of at least two of lithium difluorophosphate, lithium dioxaborate, lithium tetrafluoroborate, lithium difluorooxalato borate and lithium difluorooxalato phosphate, and the mass fraction of the additive in the electrolyte is 0.2-5.5%.

Optionally, the lithium salt additive is one or two of lithium difluorooxalate borate and lithium difluorooxalate phosphate, and the mass fraction of the lithium difluorooxalate borate and/or the lithium difluorooxalate phosphate in the electrolyte is 0.1-0.8%.

Optionally, the electrolyte includes additives including a cyclic carbonate additive and methylene methane disulfonate. Wherein the cyclic carbonate additive comprises any one or a combination of at least two of ethylene carbonate, fluoroethylene carbonate and ethylene carbonate, and the mass fraction of the methylene methane disulfonate in the electrolyte is 0.1-0.8%.

The invention further provides a lithium ion battery, which comprises a positive electrode plate, a negative electrode plate and the electrolyte.

Optionally, the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material, wherein the positive electrode active material comprises any one or a combination of at least two of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminum oxide.

Optionally, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active material, wherein the negative electrode active material comprises any one or a combination of at least two of soft carbon, hard carbon, artificial graphite, natural graphite, silicon oxygen compound, silicon carbon compound and lithium titanate.

According to the quick-charging electrolyte provided by some examples of the invention, the use of the low-viscosity diluent substituted by polyfluoro or perfluoro reduces the consumption of the high-melting-point high-viscosity cyclic carbonate solvent in the electrolyte organic solvent, so that the electrolyte forms a local high-concentration system, the viscosity of the electrolyte can be effectively reduced while the performance of the electrolyte is ensured, lithium ions in the electrolyte can be better diffused and transmitted, and the electrochemical window of the diluent is stable, so that the stability of the electrolyte on the positive and negative electrode sides is effectively improved.

The diluents provided in some examples of the invention do not participate in the solvation reaction of lithium ions in the electrolyte. By adding the diluent into the electrolyte and simultaneously reducing the content of the cyclic carbonate solvent with strong binding capacity, the diluent with sites for weak acting force in the solvation structure of lithium ions can disperse anion-cation clusters, so that the diffusion and transmission process of lithium ions in the electrolyte can be accelerated, the quick charge capacity of a lithium ion battery is further improved, and the battery is ensured to have lower impedance and higher capacity retention rate under the condition of high-rate charge.

In some examples of the invention, the diluent may self-quench upon thermal runaway of the cell of the lithium ion battery, so when such diluent is added to the electrolyte, severe reactions may be inhibited upon thermal runaway of the lithium ion battery, thereby significantly increasing the thermal runaway temperature of the lithium ion battery and thereby significantly improving its thermal safety performance. On the other hand, as the cosolvent with polarity between the diluent and the organic solvent and a slightly strong intermolecular force is added into the electrolyte, the cosolvent can also assist in dispersing the anion-cation clusters to form an easily-migrated anion-cation pair, so that the lithium ion battery has excellent thermal safety and stability and good quick charge capacity.

Detailed Description

The following description of the technical solutions in the embodiments of the present invention will be clear and complete, and it is obvious that the described embodiments are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

Unless otherwise indicated or contradicted, terms or phrases used herein have the following meanings:

and a solid electrolyte interface film (Solid Electrolyte Interface, SEI) for forming a passivation layer covering the surface of the electrode material by reacting the electrode material with the electrolyte at the solid-liquid interface during the charge and discharge of the lithium ion battery.

Reference herein to a range of values is to be construed as continuous and includes two endpoints (i.e., a minimum value and a maximum value) of the range of values and each value between the two endpoints of the range of values unless otherwise indicated. When multiple numerical ranges are provided to describe a feature or characteristic, the numerical ranges may be combined.

In the course of developing the present invention, the inventors have found by analysis that in the prior art, the linear carbonate-based solvent is poor in chemical and electrochemical stability with some electrode materials and lithium salts (especially lithium hexafluorophosphate) at high temperature, and further reacts with by-products generated from components in which the lithium salts are decomposed at high temperature, resulting in reduced gas generation. And the cyclic carbonate solvent component helping the dissociation of the lithium salt is easily oxidized on the surface of the positive electrode (especially the high-nickel positive electrode) to generate gases such as carbon dioxide and the like, so that the capacity attenuation of the battery is accelerated. On the other hand, too small an amount of the high-temperature additive added to the electrolyte also affects the stability of the solid electrolyte interface film in a high-temperature environment. The above factors together lead to the problem of poor thermal safety performance of the fast-charging electrolyte in the prior art.

In a first aspect, the invention provides an electrolyte comprising an organic solvent, a diluent, a co-solvent, and a lithium salt. The embodiment of the invention selects a carbonate solvent as an organic solvent of electrolyte, wherein the carbonate solvent comprises a cyclic carbonate solvent and a linear carbonate solvent, and specifically comprises any one or a combination of at least two of ethylene carbonate, propylene carbonate, dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate, and the mass fraction of the carbonate solvent in the electrolyte is w ₁,30％≤w₁ -65%.

The thinner is of a thinner type which can be adopted by a local high-concentration electrolyte system, particularly a thinner with certain flame retardance, and can improve the self-extinguishing property of the lithium ion battery. Preferably, the diluent is a C3-C8 perfluoro or polyfluoro substituted ether diluent. More preferably, the diluent comprises any one or a combination of at least two of ethyl nonafluorobutyl ether, perfluorobutyl methyl ether, perfluorobutyl ethyl ether.

The thinner has smaller polarity and extremely weak intermolecular acting force, so the thinner can not participate in the solvation reaction of lithium ions in the electrolyte, and meanwhile, the thinner and the lithium ions have weak intermolecular acting force such as hydrogen bonds and the like, so the coordination effect between the lithium ions and the solvent can be weakened, and the desolvation process of the lithium ions is further accelerated. By adding the diluent into the electrolyte and simultaneously reducing the content of the cyclic carbonate solvent with strong binding capacity, the diluent with sites for weak acting force in the solvation structure of lithium ions can disperse anion-cation clusters, so that the diffusion and transmission process of lithium ions in the electrolyte can be accelerated, the quick charge capacity of a lithium ion battery is further improved, and the battery is ensured to have lower impedance and higher capacity retention rate under the condition of high-rate charge.

The diluent contains more fluorine elements, and the fluorine elements can be automatically extinguished when the battery core of the lithium ion battery is in thermal runaway, so that when the diluent is added into the electrolyte, severe reaction can be inhibited in the thermal runaway test of the lithium ion battery, thereby obviously improving the thermal runaway temperature of the lithium ion battery and further obviously improving the thermal safety performance of the lithium ion battery.

In some embodiments of the present invention, when the amount of the diluent added in the electrolyte is too small, the diluent has a certain effect on optimizing the Direct Current Resistance (DCR) of the secondary battery of the lithium ion battery, but the self-extinguishing function of the electrolyte is obviously reduced, so that the thermal runaway temperature of the lithium ion battery is obviously reduced, and if too much diluent is added to the electrolyte, the DCR is seriously increased, so that the quick charging capability of the lithium ion battery is reduced. Therefore, by setting the mass fraction w ₂ of the diluent within a prescribed range, both the quick charge and the thermal safety performance of the lithium ion battery can be achieved at the same time, so that the lithium ion battery reaches a preferable level. In some embodiments of the invention, 15% to 50% w ₂%, preferably 20% to 40% w ₂%, more preferably 25% to 35% w ₂%.

Because of the large polarity difference between the diluent and the organic solvent in some embodiments of the present invention, it is difficult to stably disperse the two in the electrolyte. To solve this problem, in some examples of the present invention, a co-solvent having a polarity between that of the diluent and the organic solvent is added to the electrolyte, and the co-solvent can also assist in dispersing the clusters of anions and cations to form easily-migrating anion and cation pairs.

In some embodiments of the present invention, the cosolvent may be selected from aromatic compounds, for example, may include any one or a combination of at least two of o-difluorobenzene, m-difluorobenzene, p-difluorobenzene, 1,3, 5-trifluorobenzene, 1,2, 3-trifluorobenzene, 1,2, 4-trifluorobenzene, toluene, ethylbenzene, and trifluoromethylbenzene, and the cosolvent has a medium polarity and a low viscosity, and facilitates the diffusion of lithium ions, so that when the cosolvent is added to the electrolyte containing the diluent, an easily-migrating anion-cation pair is formed, thereby improving the overall kinetic reaction speed of the battery cell of the lithium ion battery.

In some embodiments of the present invention, when the addition amount of the cosolvent in the electrolyte is too small, the DCR of the lithium ion battery is increased, so that the quick charge capacity of the lithium ion battery is reduced, and when the addition amount of the cosolvent is too large, the mass fraction of the diluent in the electrolyte is reduced, so that the thermal safety performance of the lithium ion battery is reduced. By setting the mass fraction w ₃ of the cosolvent within a specified range, the quick charge performance and the thermal safety performance of the lithium ion battery can reach a better state at the same time. In some embodiments of the invention, 5% to 30% w ₃%. Preferably, w ₃% or less and 15% or less, more preferably, 9% or less and ₃% or less.

The mass ratio of the cosolvent to the diluent is n, namely n=w ₃/w₂, and if the lithium ion battery has stronger quick charge performance, the thermal safety performance is considered, and n is more than or equal to 1/4 and less than or equal to 2. More preferably, 0.27.ltoreq.n.ltoreq./0.4, or 0.3.ltoreq.n.ltoreq./0.35.

The carbonate solvent is composed of a cyclic carbonate solvent and a linear carbonate solvent. The cyclic carbonate-based solvent has a high dielectric constant and a high ionic conductivity, and is capable of forming a stable solid electrolyte interface film (SEI film) on the surface of the negative electrode, but has a relatively high viscosity, and is generally used as an organic solvent in an electrolyte.

In the embodiment of the invention, any one or the combination of at least two of ethylene carbonate and propylene carbonate can be selected as a cyclic carbonate solvent, and any one or the combination of at least two of dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate can be selected as a linear carbonate solvent.

If the proportion of the cyclic carbonate solvent in the carbonate solvent is too low, the dissociation capability of the electrolyte to lithium salt is insufficient, so that DCR of the lithium ion battery is increased, and the quick charge performance of the lithium ion battery is reduced, and if the proportion of the cyclic carbonate solvent in the carbonate solvent is too high, the cyclic carbonate solvent is decomposed and generated on the surface of the positive electrode, so that the interface of the electrode/electrolyte is damaged, and finally the thermal safety performance of the lithium ion battery is slightly reduced. The test results of the examples and the comparative examples in different groups show that if the mass fraction of the cyclic carbonate solvent in the carbonate solvent is 5% -20% and the rest 80% -95% are linear carbonate solvents, the interfaces of DCR and the electrode/electrolyte of the electrolyte can be kept in a relatively stable state, so that the quick charge performance and the thermal safety performance of the lithium ion battery can be ensured.

After the lithium salt in the electrolyte is dissolved in the organic solvent, a large amount of active lithium ions can be released, so that the electrolyte has better conductivity. The lithium salt can be selected according to the conventional types in the field, in the electrolyte with higher market selectivity, the contained lithium salt generally has low dissociation energy and higher solubility, the electrolyte formed after the lithium salt is dissolved can be guaranteed to have higher conductivity by low dissociation energy, so that the high rate performance of the battery is realized, the high solubility ensures that enough lithium ions are transmitted in the electrolyte, meanwhile, the lithium salt has better stability, so that the lithium salt cannot react with other components when the lithium ion battery works at high voltage and high temperature, and the electrolyte can not be continuously consumed in the subsequent circulating process if the lithium salt with good SEI film forming performance can be ensured. In an embodiment of the present invention, the lithium salt includes any one or a combination of at least two of lithium bis (fluorosulfonyl) imide, lithium bis (trifluoromethylsulfonyl) imide, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, and lithium hexafluoroarsenate. The mass fraction of the lithium salt in the electrolyte may be selected with reference to the conventional proportion in the art, and in the present invention, the mass fraction of the lithium salt in the electrolyte is 10% to 20%.

In the embodiment of the invention, in order to further improve the quick charge capacity and the thermal safety performance of the lithium ion battery, the electrolyte comprises an additive, wherein the additive comprises a cyclic carbonate additive and a lithium salt additive, the cyclic carbonate additive can promote the formation of an SEI film and effectively prevent the electrolyte from further decomposing, and can improve the low-temperature performance of the electrolyte, the lithium salt additive generally has higher electrochemical stability, can improve the conductivity of a nonaqueous electrolyte solution, can improve the low-temperature output characteristic of the lithium ion battery, can inhibit the decomposition of the positive electrode surface possibly occurring in the high-temperature cycle process, and can prevent the oxidation reaction of the electrolyte solution, thereby improving the output characteristic, the swelling characteristic and the like after high-temperature storage. The cyclic carbonate additive comprises any one or a combination of at least two of ethylene carbonate, fluoroethylene carbonate and ethylene carbonate, preferably fluoroethylene carbonate, the lithium salt additive comprises any one or a combination of at least two of lithium difluorophosphate, lithium dioxaborate, lithium tetrafluoroborate, lithium difluorooxalate borate and lithium difluorooxalate phosphate, the mass fraction of the additive in the electrolyte can be set according to the conventional proportion of the additive in the electrolyte in the field, the mass fraction of the additive in the electrolyte is 0.2% -5.5% in the embodiment of the invention. Preferably, the mass fraction of the cyclic carbonate additive is 4.5% -5.5% and the mass fraction of the lithium salt additive is 0.1% -0.8%.

According to the performance test results of the lithium ion battery in combination with some embodiments of the invention, it is found that when the lithium salt additive is added into the electrolyte as part of the additive, the lithium salt additive is preferably one or two of lithium difluorooxalato borate and lithium difluorooxalato phosphate, and the mass fraction of the lithium difluorooxalato borate and/or the lithium difluorooxalato phosphate in the electrolyte is 0.1% -0.8%, the quick charge cycle capability and the thermal runaway stability of the lithium ion battery are improved more significantly. This is because lithium difluorooxalato borate and lithium difluorooxalato phosphate can form an SEI film with better quality on the positive electrode side, and can be beneficial to the improvement of the cycling stability and the thermal safety performance of the lithium ion battery.

In some embodiments of the invention, the additives in the electrolyte include a cyclic carbonate additive and methylene methane disulfonate. Wherein the cyclic carbonate additive comprises any one or a combination of at least two of vinylene carbonate, fluoroethylene carbonate and ethylene carbonate, and preferably the mass fraction of the cyclic carbonate additive is 4.5% -5.5%. The mass fraction of the methylene methane disulfonate in the electrolyte is 0.1-0.8%, preferably 0.1-0.3%. The above embodiments show better fast charge cycle performance and thermal safety performance in performance testing of lithium ion batteries. The lithium ion battery is characterized in that after methylene methane disulfonate is added into electrolyte as an additive, on one hand, when the lithium ion battery is formed and charged, a uniform and compact SEI film is polymerized on the surface of a positive electrode, and the SEI film is used as an interface layer, has the characteristic of solid electrolyte, is an electronic insulator but is an excellent conductor of lithium ions, and lithium ions can be freely inserted and extracted through the SEI film; on the other hand, the electrolyte containing the diluent can generate an SEI film taking inorganic components as main components on the negative electrode side, can inhibit the reaction of negative active lithium, the SEI film and the electrolyte at high temperature, improves the high-temperature storage performance of the lithium ion battery, and keeps the cycling stability of the lithium ion battery.

The second aspect of the invention also provides a lithium ion battery, which comprises a positive electrode plate, a negative electrode plate, a separation film and the electrolyte in the embodiment. In the charge and discharge process of the battery, lithium ions are inserted and separated back and forth between the positive pole piece and the negative pole piece, the isolating film is arranged between the positive pole piece and the negative pole piece to play a role in isolating, and the electrolyte plays a role in conducting ions between the positive pole piece and the negative pole piece.

The positive electrode plate of the lithium ion battery provided by the embodiment of the invention comprises a positive electrode current collector and a positive electrode active material. The positive electrode current collector is generally made of a material having good electrical conductivity and mechanical strength, such as copper foil, but is not limited thereto. The positive electrode active material is used as a lithium source in a lithium ion battery, is required to provide a higher electrode potential for the lithium ion battery, maintains a stable voltage platform, and has higher ion and electron conductivities. The positive electrode active material may be selected with reference to conventional species in the art, including any one or a combination of at least two of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide. In the lithium ion battery, a lithium nickel cobalt manganese oxide positive plate is selected as a positive plate.

The negative electrode plate of the lithium ion battery comprises a negative electrode current collector and a negative electrode active material. The negative electrode current collector may be made of a material having good electrical conductivity and mechanical strength, such as copper foil, without limitation. In the research field of lithium ion batteries, the negative electrode active material needs to maintain a stable voltage platform and have a relatively stable structure during a charge-discharge reaction. The negative electrode active material may be selected with reference to conventional kinds in the art, including any one or a combination of at least two of soft carbon, hard carbon, artificial graphite, natural graphite, silicon oxygen compound, silicon carbon compound, lithium titanate, and may be selected according to actual needs by those skilled in the art. In the lithium ion battery provided by the embodiment of the invention, the negative electrode plate is a graphite-silicon composite negative electrode plate.

The isolating film is of the conventional type in the art, for example, PE or PP porous film can be selected as the isolating film, the thickness of the isolating film is 9-18 μm, the air permeability is 180s/100 mL-380 s/100mL, and the porosity is 30-50%.

The following detailed description of the technical scheme of the present invention is made by specific examples and comparative examples, and unless otherwise indicated, the raw materials and reagents used in the following examples are commercially available or may be prepared by conventional methods in the art, and the instruments used in the examples are commercially available.

Electrolyte compositions of examples and comparative examples of different groups are specifically referred to in Table 1

Example 1

In an argon atmosphere glove box with a water content of <10ppm, battery grade Ethylene Carbonate (EC), dimethyl carbonate (DMC) and ethylmethyl carbonate (EMC) were mixed in a mass ratio of 1:1:8 to form an organic solvent. The lithium salt selected in the electrolyte in Table 1 is lithium hexafluorophosphate (LiPF ₆) with a concentration of 1mol/L in the electrolyte, the diluent selected is ethyl nonafluorobutyl ether with a mass fraction w ₂ in the electrolyte of 30%, the cosolvent selected is trifluoromethyl benzene with a mass fraction w ₃ in the electrolyte of 10%, n is 1/3, and the additive is fluoroethylene carbonate (FEC) with a mass fraction of 5% in the electrolyte. And combining the components to obtain the electrolyte. In table 1, the contents of the respective components excluding the solvent are weight percentages calculated based on the total weight of the electrolyte. The electrolytes in other examples were prepared in the same manner as in this example except that the ratio of the components was specified in the table.

Example 2

This example differs from example 1 in that perfluorobutyl methyl ether was used as the diluent in the electrolyte, the remainder being identical to example 1.

Example 3

This example differs from example 1 in that w ₂ is 20%, n is 1/2, and the remainder are identical to example 1.

Example 4

This example differs from example 1 in that w ₂ is 15%, n is 2/3, and the remainder are identical to example 1.

Example 5

This example differs from example 1 in that w ₂ is 15%, w ₃ is 30%, n is 2, and the remainder are identical to example 1.

Example 6

This example differs from example 1 in that w ₂% is 20%, w ₃% is 5%, n is 1/4, and the remainder are identical to example 1.

Example 7

This example differs from example 1 in that the mass ratio of Ethylene Carbonate (EC), dimethyl carbonate (DMC) and ethylmethyl carbonate (EMC) in the organic solvent is 3:1:6, the remainder being identical to example 1.

Example 8

This example differs from example 1 in that the mass ratio of Ethylene Carbonate (EC), dimethyl carbonate (DMC) and ethylmethyl carbonate (EMC) in the organic solvent is 0.3:1:8.7, the remainder being identical to example 1.

Example 9

This example differs from example 1 in that the additive comprises fluoroethylene carbonate (FEC) and Methylene Methane Disulfonate (MMDS) in an amount of 0.2% by mass based on the total mass of the electrolyte, the remainder being identical to example 1.

Example 10

This example differs from example 1 in that the additive comprises fluoroethylene carbonate (FEC) and MMDS is added thereto in an amount of 0.5% by mass based on the total mass of the electrolyte, the remainder being identical to example 1.

Example 11

This example differs from example 1 in that lithium difluorooxalato borate (LiDFOB) was additionally added to the additive in addition to fluoroethylene carbonate (FEC), the mass fraction of LiDFOB being 0.5% based on the total mass of the electrolyte, the remainder being identical to example 1.

Example 12

This example is different from example 1 in that lithium difluorooxalato phosphate (LiDFOP) is additionally added to the additive in addition to fluoroethylene carbonate (FEC), liDFOP is 0.5% by mass based on the total mass of the electrolyte, and the remainder is identical to example 1.

Example 13

This example differs from example 1 in that lithium tetrafluoroborate (LiBF ₄) was additionally added to the additive in addition to fluoroethylene carbonate (FEC), based on the total mass of the electrolyte, the mass fraction of LiBF ₄ was 0.5%, the remainder being identical to example 1.

Comparative example 1

This comparative example differs from example 1 in that w ₂ is 60%, n is 1/6, and the remainder are identical to example 1.

Comparative example 2

This comparative example differs from example 1 in that w ₂ is 10%, n is 1, and the remainder are identical to example 1.

Comparative example 3

This comparative example differs from example 1 in that no diluent, n >2, was added to the electrolyte, the remainder being identical to example 1.

Comparative example 4

This comparative example differs from example 1 in that w ₂ is 20%, w ₃ is 50%, n is 2.5, and the remainder are identical to example 1.

Comparative example 5

This comparative example differs from example 1 in that w ₂ is 15%, no co-solvent is added to the electrolyte, and the remainder is identical to example 1.

TABLE 1 electrolyte compositions of examples and comparative examples of different groups

The electrolytes of examples 1 to 13 and comparative examples 1 to 5 were used in the preparation of lithium ion batteries, respectively.

In the embodiment of the invention, the electrochemical device is a lithium ion battery, and the lithium ion battery is a primary lithium ion battery or a secondary lithium ion battery and comprises a positive pole piece, a negative pole piece, a diaphragm positioned between the positive pole and the negative pole and electrolyte. The preparation method of the secondary lithium ion battery comprises the following steps:

(1) Preparation of lithium nickel cobalt manganese oxide positive electrode plate

Positive electrode active material (LiNi _0.8Co_0.1Mn_0.1O₂), polyvinylidene fluoride as a binder, and Super P as a conductive agent were mixed in a weight ratio of 98:1:1, and then N-methylpyrrolidone (NMP) was added to the mixed solution. Stirring the mixed system to a uniform transparent mixed solution by a vacuum stirrer to obtain anode slurry, uniformly coating the anode slurry on an aluminum foil, airing the aluminum foil at room temperature, transferring the aluminum foil to an oven for drying, and then carrying out cold pressing and slitting to obtain the anode sheet.

(2) Preparing a graphite-silicon composite negative electrode piece:

Mixing artificial graphite with a silicon-carbon compound according to a mass ratio of 9:1 to obtain a mixture serving as a negative electrode active material, super P serving as a conductive agent, sodium carboxymethylcellulose (CMC-Na) serving as a thickening agent and styrene-butadiene rubber (SBR) serving as a binder, mixing the negative electrode active material, the conductive agent, the thickening agent and the binder according to a mass ratio of 96:1:1:2, adding deionized water, fully stirring by a vacuum stirrer, uniformly stirring the mixed substances to obtain negative electrode slurry, uniformly coating the negative electrode slurry on a negative electrode current collector copper foil, airing the copper foil at room temperature, transferring the copper foil to an oven, drying, and finally cold-pressing and slitting to obtain a negative electrode plate.

(3) Preparation of electrolyte

In an argon atmosphere glove box having a water content of <10ppm, battery grade Ethylene Carbonate (EC), dimethyl carbonate (DMC), and Ethyl Methyl Carbonate (EMC) were mixed in the mass ratio shown in table 1, and the mixed solution was formed into an organic solvent. The lithium salt selected in the electrolytes of examples 1 to 13 and comparative examples 1 to 5 of the present invention was lithium hexafluorophosphate, the concentration of the lithium salt was 1mol/L, the types of diluents, co-solvents, additives and mass fractions based on the total weight of the electrolytes were all set in the above examples 1 to 13 and comparative examples 1 to 5 in Table 1, and the above components were mixed to obtain electrolytes. In Table 1, the contents of the respective components except the organic solvent are mass percentages calculated based on the total mass of the electrolyte, and the electrolytes in the different examples and comparative examples were prepared in the same manner except that the ratio of the components is specified in the table.

(4) Preparation of a separator film

A polypropylene film (PP) of 12 μm was used as a separator.

(5) Preparation of secondary lithium ion battery

And laminating the prepared positive pole piece, the isolating film and the negative pole piece in sequence, so that the isolating film is positioned between the positive pole piece and the negative pole piece to play a role in isolation. And then coating an aluminum plastic film, transferring the aluminum plastic film into a vacuum oven, drying at 120 ℃, injecting 3.0g/Ah of the prepared electrolyte, sealing, and performing electrolytic liquefaction to finally prepare the soft-packaged battery (namely the lithium ion battery) with the capacity of 1 Ah.

Performance tests were performed on lithium ion batteries assembled from the electrolytes in examples 1 to 13 and comparative examples 1 to 5, respectively, with test results shown in table 2, and test methods are as follows:

(1) Direct current impedance (DCR) test of secondary lithium ion battery

And discharging the secondary lithium ion battery to 50% of SOC (state of charge, reflecting the residual capacity of the battery) at a specified temperature, regulating the current to 4C, maintaining for 30s, and detecting the difference between the updated stable voltage and the original platform voltage, wherein the ratio of the value to the 4C current value is the direct current impedance of the battery. The DCR test result performed after the first full charge of the battery is the initial DCR of the battery.

(2) Secondary lithium ion battery cycle test

In an oven at a specified temperature (room temperature 25 ℃ or high temperature 45 ℃) the lithium ion battery is charged at a current of 3 ℃ and discharged at a current of 1 ℃, the cyclic charge and discharge are carried out in a specified potential interval, the discharge capacity of each circle is recorded, the test is ended when the battery capacity reaches 80% of the first circle capacity, and the number of battery cycles is recorded.

(3) Hot box test of secondary lithium ion battery

The secondary lithium ion battery was fully charged to 4.25V and a hot box test was performed (130 ℃ for 30min, followed by a gradual increase in temperature until thermal runaway).

For the lithium nickel cobalt manganese oxide/silicon-graphite battery, the charge and discharge cut-off voltage is 2.5-4.25V.

TABLE 2 Performance test results for different groups of examples and comparative lithium ion batteries

Analyzing the above data, the following conclusions can be drawn:

as is clear from comparative examples 1-2 and comparative example 3, the addition of diluents such as ethylnonafluorobutyl ether, perfluorobutyl methyl ether, etc. to the electrolyte can significantly raise the temperature of thermal runaway because such diluents contain more fluorine elements and can suppress the severe heat release of the reaction in thermal runaway tests.

As is clear from comparative examples 1,3, 4 and comparative examples 1-2, DCR was optimized to some extent when too little diluent was added to the electrolyte, but the thermal runaway temperature of the lithium ion battery was significantly reduced due to the reduced self-extinguishing function of the electrolyte. When the amount of the diluent is too large, DCR is seriously deteriorated, resulting in a decrease in the fast charge capacity of the lithium ion battery. Therefore, the addition amount of the diluent should be controlled within a specified range so as to achieve both the quick charge and the thermal safety performance of the lithium ion battery. Within the optimum amount range, increasing the amount of diluent slightly deteriorates the DCR and fast charge performance of the lithium ion battery, but improves the thermal safety performance thereof, because such diluent has a larger viscosity and no ability to dissociate lithium salt, resulting in a certain reduction in the electrolyte conductivity.

As is clear from comparative examples 1, 4 to 6 and comparative examples 4 and 5, when the addition of the cosolvent to the electrolyte containing the diluent is too small, DCR of the lithium ion battery is greatly increased and its fast charge capacity is lowered, because the cosolvent having a medium polarity for assisting in forming an easily-migrating anion-cation pair is absent from the electrolyte at this time, so that the kinetics of the battery cell as a whole is deteriorated. And when the cosolvent is added into the electrolyte too much, the diluent ratio is reduced, so that the thermal safety performance of the lithium ion battery is deteriorated. Within the optimum amount range, increasing the amount of co-solvent will slightly improve DCR and fast-fill performance, as such co-solvents have low viscosity and thus can assist in the diffusion of lithium ions.

As is clear from comparative examples 1, 7 and 8, too low a proportion of the cyclic carbonate solvent in the carbonate solvent results in insufficient dissociation capability of the electrolyte to the lithium salt, and further, DCR of the lithium ion battery has a certain deterioration performance, while too high a proportion of the cyclic carbonate solvent in the carbonate solvent results in decomposition and gas generation due to insufficient stability of the cyclic carbonate organic solvent on the surface of the positive electrode, further, interface stability between the electrode and the electrolyte is deteriorated, and finally, thermal safety performance of the lithium ion battery is slightly reduced. More preferably, the cyclic carbonate solvent, such as ethylene carbonate and/or propylene carbonate, comprises 5% to 20% by mass of the carbonate solvent.

As can be seen from comparative examples 1, 9 and 10, when MMDS is added as a part of the additive in the electrolyte, on one hand, the surface of the positive electrode is polymerized to generate a uniform and compact SEI film when the lithium ion battery is formed and charged, on the other hand, the electrolyte containing the diluent generates an SEI film mainly composed of inorganic components on the negative electrode side, and the reaction of the negative electrode activity with the SEI film and the electrolyte at high temperature can be inhibited, so that the high-temperature storage performance of the lithium ion battery is improved, the stability of the rapid charging cycle of the lithium ion battery is maintained, and therefore, the rapid charging cycle of the lithium ion battery added with MMDS has better performance, and meanwhile, the interfaces of the electrodes of the positive electrode and the negative electrode and the electrolyte have more stable performance, so that the thermal safety performance of the lithium ion battery can be improved to a certain extent.

Comparative examples 1, 11-13 show that the improvement in the fast charge cycle capability and the increase in the thermal case runaway temperature of lithium ion batteries are more pronounced with LiDFOB and LiDFOP than with LiBF ₄. This is because the quality of the film formation of the LiDFOB and LiDFOP on the positive electrode side is superior to that of LiBF ₄, which is advantageous in the improvement of the cycle stability and the improvement of the thermal safety, so that the lithium salt additive is preferably LiDFOB or LiDFOP.

The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather is intended to cover all modifications, equivalents, and alternatives falling within the spirit and principles of the invention.

Claims

1. An electrolyte, characterized in that it comprises:

An organic solvent, wherein the organic solvent comprises a carbonate solvent, and the mass fraction of the carbonate solvent in the electrolyte is w ₁ , 30%≤w ₁ ≤65%;

A lithium salt dissolved in the organic solvent;

a diluent which does not participate in the solvation reaction of lithium ions in the organic solvent and has flame retardancy, wherein the mass fraction of the diluent in the electrolyte is w ₂ , 15%≤w ₂ ≤50%;

A co-solvent, wherein the co-solvent is an aromatic compound, the polarity of the co-solvent is between the diluent and the organic solvent, and the mass fraction of the co-solvent in the electrolyte is w ₃ , 5%≤w ₃ ≤30%.

2 . The electrolyte according to claim 1 , wherein the mass ratio of the auxiliary solvent to the diluent is n, n=w ₃ /w ₂ , and n satisfies the following relationship: 1/4≤n≤2.

3. The electrolyte according to claim 1, characterized in that the diluent is a C3-C8 perfluorinated or polyfluorinated ether diluent.

4. The electrolyte according to claim 1, characterized in that the diluent is any one of ethyl nonafluorobutyl ether, perfluorobutyl methyl ether, and perfluorobutyl ethyl ether, or a combination of at least two thereof;

The co-solvent is any one of o-difluorobenzene, m-difluorobenzene, p-difluorobenzene, 1,3,5-trifluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, toluene, ethylbenzene, and trifluoromethylbenzene, or a combination of at least two thereof.

5 . The electrolyte according to claim 1 , wherein the carbonate solvent consists of a cyclic carbonate solvent and a linear carbonate solvent, and the mass fraction of the cyclic carbonate solvent in the carbonate solvent is 5%-20%.

6 . The electrolyte according to claim 5 , wherein the cyclic carbonate solvent is ethylene carbonate and/or propylene carbonate.

7. The electrolyte according to claim 1, characterized in that the lithium salt comprises any one of lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, and lithium hexafluoroarsenate, or a combination of at least two thereof, and the mass fraction of the lithium salt in the electrolyte is 10%-20%.

8. The electrolyte according to claim 1, further comprising an additive, wherein the additive comprises:

A cyclic carbonate additive, wherein the cyclic carbonate additive includes any one of vinylene carbonate, fluoroethylene carbonate, and vinyl ethylene carbonate, or a combination of at least two thereof; and

A lithium salt additive, wherein the lithium salt additive includes any one of lithium difluorophosphate, lithium dioxalatoborate, lithium tetrafluoroborate, lithium difluorooxalatoborate, and lithium difluorooxalatophosphate, or a combination of at least two thereof;

The mass fraction of the additive in the electrolyte is 0.2%-5.5%.

9. The electrolyte according to claim 8, characterized in that the lithium salt additive is one or both of the lithium difluorooxalatoborate and the lithium difluorooxalatophosphate, and the mass fraction of the lithium difluorooxalatoborate and/or the lithium difluorooxalatophosphate in the electrolyte is 0.1%-0.8%.

10. The electrolyte according to claim 1, further comprising an additive, wherein the additive comprises:

Methylene methanedisulfonate, wherein the mass fraction of the methylene methanedisulfonate in the electrolyte is 0.1%-0.8%.

11. A lithium-ion battery, characterized in that the lithium-ion battery comprises a positive electrode sheet, a negative electrode sheet and the electrolyte according to any one of claims 1 to 10.

12. The lithium-ion battery according to claim 11, wherein the positive electrode sheet comprises:

a positive electrode current collector; and

The positive electrode active material includes any one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide or lithium nickel cobalt aluminum oxide, or a combination of at least two thereof.

13. The lithium-ion battery according to claim 11, wherein the negative electrode plate comprises:

a negative electrode current collector; and

The negative electrode active material includes any one of soft carbon, hard carbon, artificial graphite, natural graphite, silicon, silicon oxide, silicon carbon compound, and lithium titanate, or a combination of at least two thereof.