CN105845984A - Lithium ion battery electrolyte and lithium ion battery using same - Google Patents

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a high-voltage quick-charging lithium ion battery electrolyte and a lithium ion battery using the same; the electrolyte comprises lithium salt, a non-aqueous organic solvent and an additive, wherein the additive comprises fluoroethylene carbonate, a nitrile compound and a compound with a structure shown in a formula I; wherein R in formula I₁、R₂、R₃、R₄、R₅Each independently selected from hydrogen, amide, alkyl or alkoxy having 1 to 3 carbon atoms, and R₁、R₂、R₃、R₄、R₅Contains at least one amide group. Compared with the prior art, the lithium ion battery electrolyte has the advantages that the lithium ion battery electrolyte has excellent infiltration effect and quick charge performance under the condition of high voltage and has the comprehensive performance of long cycle life and low high-temperature storage expansion rate through the synergistic effect generated by fluoroethylene carbonate, nitrile compounds and amide pyridine compounds with the structure shown in the formula I.

Description

Lithium-ion battery electrolyte and lithium-ion battery using the same

technical field

The invention relates to the technical field of lithium-ion batteries, in particular to a high-voltage fast-charging lithium-ion battery electrolyte and a lithium-ion battery using the electrolyte.

Background technique

Since its commercialization in 1991, lithium-ion batteries have rapidly been widely used in consumer and power terminals due to their advantages such as high capacity density, high working voltage, low pollution in the manufacturing process, and clean and pollution-free use process. From primary lithium metal batteries to secondary rechargeable lithium-ion batteries such as cylinders, aluminum shells, and soft packs, lithium batteries are also showing an increasingly diverse product pattern, and highly customized, controllable shapes, and wide performance are gradually becoming a reality. .

Although in emerging consumer fields such as mobile phones and tablet computers that have emerged in recent years, lithium-ion batteries have shown great advantages due to their high capacity density and long cycle life. As a result, the disadvantages of existing lithium-ion battery positive and negative electrode materials and conventional battery design continue to emerge. In order to solve this problem, most of the industry obtains a larger volumetric energy density by increasing the charging upper limit voltage of the positive electrode or increasing the compaction density of the negative electrode diaphragm. In addition, on the basis of not changing the charging frequency of the original battery, the purpose of fast charging can be achieved by shortening the time required for each charging.

Among them, for fast-charging lithium-ion batteries, most manufacturers use existing active materials while reducing the surface density or compaction density of the positive and negative electrodes; some manufacturers use positive and negative active materials with better kinetic properties, such as Positive electrode materials with large particles and low specific surface area, and artificial graphite coated with soft carbon. However, these methods undoubtedly bring negative effects such as shortened cycle life and reduced energy density, so it is necessary to find a reasonable solution from the electrolyte side.

The reason why the electrolyte is so critical is that during the fast charging process of the battery, lithium ions quickly escape from the positive electrode into the electrolyte, and then pass through the separator and enter the negative electrode for lithium insertion. The rapid migration of a large number of lithium ions requires the electrolyte to have a high Kinetic performance, with smaller mass transfer resistance in the mass transfer process, so the electrolyte needs to have better wettability, lower viscosity and lower lithium ion transport resistance.

However, the existing fast-charging lithium-ion battery electrolyte generally uses a low-viscosity, high-conductivity solvent combined with a high lithium salt concentration technical solution, and this solution has the following defects: first, the cost advantage is not obvious; second, the battery produced The high temperature storage effect is poor and the cycle life is short.

In view of this, it is indeed necessary to optimize the electrolyte for the existing fast-charge ion battery, so that it not only has fast-charge performance, but also has good high-temperature storage and cycle performance.

Contents of the invention

One of the purposes of the present invention is to provide a battery that not only has excellent wetting effect, excellent fast charging performance, but also has good cycle performance in view of the shortcomings of high-temperature storage and poor cycle performance of the electrolyte used in current fast-charge lithium-ion batteries. Lithium-ion battery electrolyte with high temperature storage performance.

In order to achieve the above object, the present invention adopts the following solutions:

An electrolyte solution for a lithium-ion battery, comprising a lithium salt, a non-aqueous organic solvent and an additive, the additive comprising fluoroethylene carbonate, a nitrile compound and a compound having a structure shown in formula I;

Wherein, R ₁ , R ₂ , R ₃ , R ₄ , and R ₅ in formula I are independently selected from hydrogen, amido, alkyl or alkoxy with 1 to 3 carbon atoms, and R ₁ , R _2. R ₃ , R ₄ , and R ₅ contain at least one amide group.

In the present invention, through the synergistic effect of fluoroethylene carbonate, nitrile compounds and compounds with the structure shown in formula I, the battery using the electrolyte not only has excellent wetting effect, low electrochemical impedance, and fast The charging effect is good, and it has excellent cycle and high temperature storage performance. Among them, during the first charging process, the fluoroethylene carbonate is reduced on the surface of the graphite negative electrode to form a thin and stable SEI film, which effectively isolates the solvent from the active sites of the graphite and prevents the electrolyte from being reduced during the use of the battery. Ethylene carbonate can also improve the wettability of the electrolyte to the negative electrode membrane; the nitrile compound can complex with the transition metal atoms in the positive active material to form a positive passivation film, which separates the solvent from the positive active material. The site is effectively isolated to slow down the severe oxidation and decomposition of the electrolyte during the cycle and affect the cycle life; in addition, the passivation film can effectively curb the dissolution of transition metals in high-temperature storage environments, so the high-temperature storage performance of the electrolyte is effectively improved ; The pyridine compound with the structure shown in formula I, on the one hand, the amide group it contains can reduce the overall viscosity of the electrolyte, and achieve good infiltration of the negative electrode diaphragm, so as to achieve excellent capacity and fast charging performance; On the one hand, the pyridine heterocycle contained in it can complex with the transition metal elements in the positive electrode active material to further passivate the positive electrode to protect the electrolyte solvent from oxidative decomposition, thereby obtaining excellent cycle performance.

Preferably, the mass percentage of the fluoroethylene carbonate in the electrolyte is 1% to 8%; the mass percentage of the nitrile compound in the electrolyte is 0.05% to 5%; the The mass percent content of the compound represented by formula I in the electrolyte is 0.01%-8%, preferably 0.5%-5%.

Preferably, the lithium salt is at least one of lithium hexafluorophosphate, lithium bisoxalate borate, lithium difluorooxalate borate, lithium bisfluorosulfonyl imide, lithium tetrafluoroborate and lithium bistrifluoromethanesulfonyl imide; The mass percent content of the lithium salt in the electrolyte is 12%-20%.

Preferably, the non-aqueous organic solvent is selected from ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl propionate, propyl propionate, methyl propyl carbonate, ethyl acetate, propyl acetate One or more of esters, methyl acetate, methyl butyrate, ethyl butyrate, propyl butyrate, tetrahydrofuran, dioxane, γ-butyrolactone; the non-aqueous organic solvent in the electrolyte The mass percentage content in is 65%～85%. The above-mentioned non-aqueous organic solvent has a high decomposition potential, and has good thermal stability and electrochemical stability at high temperature and high voltage, thereby providing stable electrical properties for 4.4V and above high-voltage fast-charge lithium-ion batteries. electrochemical environment.

Preferably, the nitrile compound is at least one of adiponitrile, succinonitrile, glutaronitrile, pimelonitrile, 2-methylglutaronitrile and 1,3,6-hexanetrinitrile. The nitrile compounds listed can improve the stability of the positive electrode material of the lithium-ion battery at high voltage, inhibit the oxidation and decomposition of the electrolyte on the surface of the positive electrode, and improve the high-temperature storage performance and cycle performance of the high-voltage lithium-ion battery.

Preferably, the compound having the structure shown in formula I is selected from 2,6-dipyridinedicarboxamide (structural formula shown in formula I-1), 2-pyridinecarboxamide (structural formula shown in formula I-2) , one or more of 3-pyridinecarboxamide (structural formula shown in formula I-3) and 4-pyridinecarboxamide (structural formula shown in formula I-4).

The second object of the present invention is to provide a lithium-ion battery using the above-mentioned electrolyte, which not only has excellent rate cycle And fast charging performance, while ensuring good high-temperature storage and cycle performance.

A lithium ion battery prepared by using the above-mentioned lithium ion battery electrolyte, comprising a positive pole piece, a negative pole piece, a diaphragm arranged between the positive pole piece and the negative pole piece, and an electrolyte, the positive pole piece The sheet includes a positive electrode collector and a positive electrode membrane coated on the surface of the positive electrode collector, and the negative electrode sheet includes a negative electrode collector and a negative electrode membrane coated on the surface of the negative electrode collector.

Wherein, the positive electrode membrane includes a positive electrode active material, a conductive agent and a binder, and the compacted density of the positive electrode membrane is ≥4.0 g/cm ³ .

The negative electrode membrane includes negative electrode active material, conductive agent and binder, and the surface density of the negative electrode membrane is ≥80g/m ² , and the compacted density is ≥1.55g/cm ³ .

The charging cut-off voltage of the lithium ion battery is 4.4-4.7V.

The present invention has at least the following beneficial effects:

1) a kind of lithium ion battery electrolyte of the present invention, comprises lithium salt, non-aqueous organic solvent and additive,

The additives include fluoroethylene carbonate, nitrile compounds and amide pyridine compounds with the structure shown in formula I; wherein, fluoroethylene carbonate forms an SEI film on the surface of the graphite negative electrode to ensure cycle life, while fluoroethylene carbonate Ester can improve the wettability of the electrolyte, reduce the transfer resistance (electrochemical impedance) of effective lithium between graphite layers, thereby effectively improving the fast charging performance of the battery; nitrile compounds, complexed with transition metals in the positive active material function, inhibit the dissolution of transition metals under high-temperature storage conditions, absorb trace moisture in the digestion battery, reduce the generation of phosphorus pentafluoride and flatulence, and improve high-temperature performance; The pyridine heterocycle contained in it coordinates with the transition metal in the positive active material to protect the solvent from oxidative decomposition and improve high-temperature storage and cycle performance; on the other hand, the amide group contained in it can reduce the viscosity of the electrolyte and improve the electrolyte Wetting the pole piece, and realizing the rapid intercalation and deintercalation of lithium, improving the fast charging performance;

2) The non-aqueous organic solvent system selected by the present invention includes solvents with high dielectric constants such as ethylene carbonate and propylene carbonate, which are beneficial to the dissolution of lithium salts, the improvement of battery high temperature and cycle performance; in addition, it contains low viscosity and The solvent component with a wide electrochemical window can not be decomposed by the positive electrode with high potential, and the low viscosity can meet the requirements of the electrolyte to infiltrate the electrode sheet;

3) The present invention adjusts the synergistic effect produced by the ratio of fluoroethylene carbonate, nitrile compounds and amidopyridine compounds having the structure shown in formula I, so that the lithium-ion battery prepared using the electrolyte can operate at 4.4V and Under the above high voltage conditions, not only the wetting effect is excellent, the fast charging performance is excellent, but also the comprehensive performance of long cycle life and low expansion at high temperature storage is obtained, which has broad application prospects.

Description of drawings

FIG. 1 is a diagram of the cycle performance of lithium-ion batteries prepared in Examples 1-8 and Comparative Examples 1-4 of the present invention.

detailed description

The present invention and its beneficial effects will be described in further detail below in conjunction with specific embodiments and accompanying drawings, but the specific embodiments of the present invention are not limited thereto.

Example 1

Electrolyte preparation:

In a glove box filled with argon (moisture content <10ppm, oxygen content <1ppm), ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate were mixed at 25:10:30:10 : The mass fraction of 25 is mixed evenly and constantly stirred, and in the mixed solvent, slowly adding mass fraction is 15% LiPF ₆ , then adding mass fraction is 3% fluoroethylene carbonate, mass fraction is 1% adiponitrile and 2,6-dipyridinedicarboxamide with a mass fraction of 0.5% was stirred until it was completely dissolved to obtain the lithium-ion battery electrolyte of Example 1.

Preparation of lithium-ion batteries:

After the positive electrode active material lithium cobaltate, the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) are fully stirred and mixed in the N-methylpyrrolidone solvent system at a mass ratio of 95:3:2, they are coated on Drying on the Al foil and cold pressing to obtain the positive electrode sheet, the compacted density of which is 4.0 g/cm ³ .

Negative electrode active material graphite, conductive agent Super P, binder styrene-butadiene rubber (SBR), thickener carbon methyl cellulose sodium (CMC) were mixed in a deionized water solvent system according to the mass ratio of 95:2:2:1 After fully stirring and mixing evenly, it was coated on Cu foil, dried, and cold-pressed to obtain a negative electrode sheet with an areal density of 80 g/m ² and a compacted density of 1.55 g/cm ³ .

The PP-PE-PP film was used as the base film (12 μm) and the nano-alumina coating (3 μm) was coated on the base film as the separator.

Stack the positive pole piece, diaphragm, and negative pole piece in order, so that the diaphragm is in the middle of the positive and negative pole pieces to play the role of isolation, and wind up to get the bare cell. Put the bare cell in the outer packaging, inject the prepared electrolyte, and go through the processes of packaging, shelving, formation, aging, secondary packaging, and capacity separation to obtain a lithium-ion battery with a model number of 504848.

Example 2

Different from Example 1 is the preparation of electrolyte:

In a glove box filled with argon (moisture content <10ppm, oxygen content <1ppm), ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate were mixed at 25:10:30:10 : The mass fraction of 25 is mixed evenly and constantly stirred, and in the mixed solvent, slowly adding mass fraction is 15% LiPF ₆ , then adding mass fraction is 3% fluoroethylene carbonate, mass fraction is 1% adiponitrile and 2,6-dipyridinedicarboxamide with a mass fraction of 3% was stirred until it was completely dissolved to obtain the lithium-ion battery electrolyte of Example 2.

The rest are the same as in Embodiment 1, and will not be repeated here.

Example 3

Different from Example 1 is the preparation of electrolyte:

In a glove box filled with argon (moisture content <10ppm, oxygen content <1ppm), ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate were mixed at 25:10:30:10 : The mass fraction of 25 is mixed evenly and constantly stirred, and in the mixed solvent, slowly adding mass fraction is 15% LiPF ₆ , then adding mass fraction is 3% fluoroethylene carbonate, mass fraction is 1% adiponitrile and The 2-pyridinecarboxamide with a mass fraction of 0.5% was stirred until completely dissolved to obtain the lithium-ion battery electrolyte of Example 3.

The rest are the same as in Embodiment 1, and will not be repeated here.

Example 4

Different from Example 1 is the preparation of electrolyte:

In a glove box filled with argon (moisture content <10ppm, oxygen content <1ppm), ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate were mixed at 25:10:30:10 : The mass fraction of 25 is mixed evenly and constantly stirred, and in the mixed solvent, slowly adding mass fraction is 15% LiPF ₆ , then adding mass fraction is 3% fluoroethylene carbonate, mass fraction is 1% adiponitrile and The 2-pyridinecarboxamide with a mass fraction of 3% was stirred until completely dissolved to obtain the lithium-ion battery electrolyte of Example 4.

The rest are the same as in Embodiment 1, and will not be repeated here.

Example 5

Different from Example 1 is the preparation of electrolyte:

In a glove box filled with argon (moisture content <10ppm, oxygen content <1ppm), ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate were mixed at 25:10:30:10 : The mass fraction of 25 is mixed evenly and constantly stirred, and in the mixed solvent, slowly adding mass fraction is 15% LiPF ₆ , then adding mass fraction is 3% fluoroethylene carbonate, mass fraction is 1% adiponitrile and The 3-pyridinecarboxamide with a mass fraction of 0.5% was stirred until completely dissolved to obtain the lithium-ion battery electrolyte of Example 5.

The rest are the same as in Embodiment 1, and will not be repeated here.

Example 6

Different from Example 1 is the preparation of electrolyte:

In a glove box filled with argon (moisture content <10ppm, oxygen content <1ppm), ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate were mixed at 25:10:30:10 : The mass fraction of 25 is mixed evenly and constantly stirred, and in the mixed solvent, slowly adding mass fraction is 15% LiPF ₆ , then adding mass fraction is 3% fluoroethylene carbonate, mass fraction is 1% adiponitrile and Stir the 3-pyridinecarboxamide with a mass fraction of 3% until it is completely dissolved to obtain the lithium-ion battery electrolyte of Example 6.

The rest are the same as in Embodiment 1, and will not be repeated here.

Example 7

Different from Example 1 is the preparation of electrolyte:

In a glove box filled with argon (moisture content <10ppm, oxygen content <1ppm), ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate were mixed at 25:10:30:10 : The mass fraction of 25 is mixed evenly and constantly stirred, and in the mixed solvent, slowly adding mass fraction is 15% LiPF ₆ , then adding mass fraction is 3% fluoroethylene carbonate, mass fraction is 1% adiponitrile and The 4-pyridinecarboxamide with a mass fraction of 0.5% was stirred until completely dissolved to obtain the lithium-ion battery electrolyte of Example 7.

The rest are the same as in Embodiment 1, and will not be repeated here.

Example 8

Different from Example 1 is the preparation of electrolyte:

In a glove box filled with argon (moisture content <10ppm, oxygen content <1ppm), ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate were mixed at 25:10:30:10 : The mass fraction of 25 is mixed evenly and constantly stirred, and in the mixed solvent, slowly adding mass fraction is 15% LiPF ₆ , then adding mass fraction is 3% fluoroethylene carbonate, mass fraction is 1% adiponitrile and Stir the 4-pyridinecarboxamide with a mass fraction of 3% until it is completely dissolved to obtain the lithium-ion battery electrolyte of Example 8.

The rest are the same as in Embodiment 1, and will not be repeated here.

Comparative example 1:

Different from Example 1 is the preparation of electrolyte:

In a glove box filled with argon (moisture content <10ppm, oxygen content <1ppm), ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate were mixed at 25:10:30:10 : The mass ratio of 25 is mixed evenly and constantly stirred, slowly adding the LiPF ₆ that the mass fraction is 15% in the mixed solvent, then adding the fluoroethylene carbonate that the mass fraction is 1%, and stirring until completely dissolved, promptly obtains comparative example 1 lithium-ion battery electrolyte.

The rest are the same as in Embodiment 1, and will not be repeated here.

Comparative example 2:

Different from Example 1 is the preparation of electrolyte:

In a glove box filled with argon (moisture content <10ppm, oxygen content <1ppm), ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate were mixed at 25:10:30:10 : The mass ratio of 25 is mixed evenly and constantly stirred, and in the mixed solvent, slowly adding mass fraction is 15% LiPF ₆ , then adding mass fraction is 5% fluoroethylene carbonate, and stirring until completely dissolved, promptly obtains comparative example 2 lithium-ion battery electrolyte.

The rest are the same as in Embodiment 1, and will not be repeated here.

Comparative example 3:

Different from Example 1 is the preparation of electrolyte:

In a glove box filled with argon (moisture content <10ppm, oxygen content <1ppm), ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate were mixed at 25:10:30:10 : The mass ratio of 25 is mixed evenly and constantly stirred, slowly adding mass fraction in mixed solvent is 15% LiPF ₆ , then adding mass fraction is 3% fluoroethylene carbonate and mass fraction is 1% adiponitrile, Stir until completely dissolved to obtain the lithium-ion battery electrolyte of Comparative Example 3.

The rest are the same as in Embodiment 1, and will not be repeated here.

Comparative example 4:

Different from Example 1 is the preparation of electrolyte:

In a glove box filled with argon (moisture content <10ppm, oxygen content <1ppm), ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, ethyl acetate were mixed at 25:10:30:10 : The mass ratio of 25 is mixed evenly and constantly stirred, and in the mixed solvent, slowly adding mass fraction is 15% LiPF ₆ , then adding mass fraction is 3% fluoroethylene carbonate and 3% adiponitrile, and stirs until completely Dissolving, promptly obtains the lithium-ion battery electrolyte of comparative example 4.

The rest are the same as in Embodiment 1, and will not be repeated here.

Fast charging performance tests were performed on the lithium-ion batteries prepared in Examples 1-8 and Comparative Examples 1-4 respectively. The test results are shown in Table 1.

Table 1 Fast charging performance test results of Examples 1-8 and Comparative Examples 1-4 in an environment of 25±1°C

Note: The constant current time percentage refers to the percentage of the constant current charging time to the total charging time; the constant current capacity percentage refers to the percentage of the constant current charging capacity to the total charging capacity.

In addition, in order to further reflect the advantages of the present invention, the following performance tests were performed on the same batch of batteries prepared in Examples 1-8 and Comparative Examples 1-4:

Normal temperature 2C/1C cycle test: Charge the battery at 25±1℃ with a constant current at a rate of 2C to a limit voltage of 4.4V, then charge at a constant voltage to a cut-off current of 0.05C, leave it for 5 minutes, and then discharge it at a constant current with a discharge rate of 1C , the discharge cut-off voltage is 3.0V; finally, the cycle test is performed, and the number of cycles is set to more than 400 times. The test results are shown in Figure 1.

High temperature storage at 85°C for 6h test: charge the battery with a constant current of 0.2C rate to the limit voltage of 4.4V, then charge the battery at a constant voltage to the cut-off current of 0.02C, leave it for 5 minutes, then discharge it at a constant current of 0.2C rate to the cut-off voltage of 3.0V, record The discharge capacity is the initial discharge capacity, and the thickness and internal resistance of the battery in the initial state are measured; the fully charged battery is placed in an oven at 85°C, stored at a constant temperature of 85±2°C for 6 hours, and tested immediately after storage The thickness of the battery at high temperature is cooled at room temperature for 2 hours, and the internal resistance is tested. The remaining capacity is tested by constant current discharge at a rate of 0.2C, and the recovery capacity is tested at a rate of 0.2C. Calculate the thickness expansion rate, internal resistance increase rate, and remaining Capacity Percentage, Recovery Capacity Percentage. The relevant data obtained from the test are shown in Table 2.

Table 2 Storage performance test results of Examples 1-8 and Comparative Examples 1-4 at 85°C

The above test results are analyzed in detail below.

From the performance test results of Comparative Example 1 and Comparative Example 2, it can be found that increasing the amount of fluoroethylene carbonate is beneficial to improve the fast charging performance of the battery. This is because: on the one hand, fluoroethylene carbonate itself is a kind of carbonate organic compound, and it presents a liquid state in a slightly higher than room temperature environment, and when its dielectric constant reaches a certain level, it will dissolve the ionization of lithium salt It plays a promoting role, and the slightly balanced viscosity of the electrolyte will facilitate its infiltration of the pole piece; on the other hand, fluoroethylene carbonate contains F, and its strong electronegativity can dilute the charge density of the ethylene carbonate ring to form a more The stable structure reduces the van der Waals interaction with the electrolyte in the battery and the binder in the pole piece diaphragm, further increasing the wetting effect of the electrolyte on the pole piece. The combination of the above two aspects is beneficial to reduce the electrochemical impedance of electron transfer embedded in the pole piece, and reduce the mass transfer impedance of electron transfer in the electrolyte, thereby shortening the charging time of the battery. However, it should be noted that from the results of the storage test at 85°C in the fully charged state, fluoroethylene carbonate not only did not provide beneficial help, but increased the battery expansion, and the internal resistance increased and the charge retention also performed poorly. This is due to the failure mechanism of lithium-ion batteries stored at 85°C in a fully charged state. Lithium hexafluorophosphate is mostly used as the main electrolyte in the electrolyte, and lithium hexafluorophosphate has begun to decompose violently at 85°C, producing phosphorus pentafluoride and lithium fluoride. , and phosphorus pentafluoride is a strong Lewis acid, which is very easy to destroy the SEI film formed when the battery is charged for the first time in the battery, and decomposes to produce gases such as carbon dioxide and ethylene. Therefore, the reason for the poor high-temperature storage performance of Comparative Example 1 and Comparative Example 2 is that fluoroethylene carbonate, as a kind of ethylene carbonate organic compound, also has the effect of forming an SEI film, but the formed SEI film is Organic alkyl lithium carbonate with low rigidity; and under the storage environment of 85°C in the fully charged state, on the one hand, the SEI film itself will decompose due to its low rigidity; on the other hand, phosphorus pentafluoride produced by the decomposition of lithium hexafluorophosphate is also stable to this The weak SEI film causes great damage and generates gas.

From the test results of Comparative Example 3 and Comparative Example 4, it can be seen that: the high-temperature storage performance of Comparative Example 4 is obviously better than that of Comparative Example 3, but the fast charging performance and cycle performance are not as good as Comparative Example 3. From the analysis of the injected electrolytes in the two comparative examples, it can be seen that comparative example 4 only added 2 wt% of adiponitrile on the basis of comparative example 3. Nitriles, as a class of compounds containing carbon-nitrogen triple bonds and lacking electrons, can be added to the electrolyte as an additive to achieve a good effect of removing acid and water, especially at 85°C when the battery is fully charged. Because there is a small amount of moisture in the battery, the decomposition reaction of lithium hexafluorophosphate will proceed in the positive direction during storage at 85°C, and more phosphorus pentafluoride will be generated to cause the battery to fail. And the added nitrile compound can reduce the trace water content therein, effectively controlling the further progress of failure reaction. In addition, because the nitrile compound contains unsaturated bonds, it is very easy to coordinate with the transition metal in the positive electrode active material, effectively preventing the transition metal from leaching from the active material structure, and reducing the risk of oxidation-reduction reactions between the leached metal and the electrolyte solvent. probability. However, although nitrile compounds have remarkable high-temperature effects, due to their own structural characteristics, on the one hand, they easily hinder the battery from forming an SEI film on the first charge, and its compatibility with graphite negative electrodes is poor; The mass transfer impedance rises significantly, so the comprehensive result leads to a great reduction in the fast charging and cycle performance of the battery.

Simply dissolving lithium hexafluorophosphate in ethylene carbonate or chain carboxylate, or adding additives such as fluorocarbonate and nitrile compounds to the electrolyte is a typical solution for mass production of electrolytes on the market. Although by adjusting the ratio of each component to find the best combination of performance, good results can be achieved. However, for fast-charging lithium-ion batteries, through the test results of Comparative Examples 1 to 4 and the above analysis, it is not difficult to find that the effect of conventional additive combinations is limited, and it is difficult to further improve the fast-charging and cycle performance.

Therefore, the present invention introduces pyridine compounds containing amide groups in Examples 1-8. The structure of the compound contains a pyridine heterocycle, and at the same time, the amide group and pyridine heterocycle produce a conjugation effect. Among them, the amide group contained in it can reduce the viscosity of the electrolyte, improve the infiltration of the electrolyte on the pole piece, and realize the rapid intercalation and deintercalation of lithium, and improve the fast charging performance; The transition metals in the positive electrode active material are coordinated to effectively prevent a large amount of transition metals from leaching out; and through the film formation of the positive electrode, the solvent in the electrolyte is effectively isolated from the positive electrode active site, so that the high-temperature storage and cycle performance of the battery are effectively improved. promote. From the cycle and high-temperature storage test results of Examples 1-8, it can be seen that the closer the position of the amide group in the pyridine ring is to the nitrogen, the more obvious the performance is, because the conjugation effect produced by this structure is relatively strong. Simultaneous analysis of the fast charging effect is also based on the conjugation effect so that amide pyridine additives such as 2,6-dipyridinedicarboxamide, 2-pyridinecarboxamide, 3-pyridinecarboxamide, and 4-pyridinecarboxamide have a better effect on the pole piece. Strong wettability effectively reduces the electrochemical impedance of the pole piece, which is conducive to the unimpeded transfer of effective lithium between graphite layers; thus achieving a larger charging capacity in a shorter period of time, and the high-rate charging efficiency is also significantly improved.

It is not difficult to judge from the above analysis that the addition of fluoroethylene carbonate makes the battery form a relatively stable SEI film during the first charging process to ensure cycle performance, and can improve the infiltration of the electrolyte on the pole piece to make the battery fast charge The effect is remarkable; however, the SEI film formed by fluoroethylene carbonate is easy to decompose and produce gases such as carbon dioxide and ethylene under high temperature storage conditions, which will cause battery inflation. After the introduction of nitrile compounds, although the inflation of fluoroethylene carbonate in high temperature storage is effectively controlled, at the same time, due to the high viscosity and high impedance characteristics of nitrile compounds, the electrochemical impedance and mass transfer impedance of the battery become larger. And it will hinder the formation of battery SEI film during the first charging process. Therefore, the present invention tries to add amidopyridine additives in the fluoroethylene carbonate and nitrile compound additive system, and finds that the effect is remarkable. This is mainly due to the fact that the amidopyridine additives have a film-forming function on the positive electrode, which can effectively prevent the gas swelling caused by fluoroethylene carbonate under high temperature storage; Chemical resistance and fast charging performance are improved, thus effectively offsetting the defects introduced by nitrile compounds. Therefore, the multiple additives of the present invention cooperate with each other and complement each other, and finally make the overall performance of the battery excellent.

Based on the above analysis, the present invention makes the 4.4V And above high-voltage fast-charge lithium-ion batteries have the comprehensive performance of short-term charging, low expansion rate of full-charge high-temperature storage, and excellent cycle performance. Compared with the general environment of existing common battery designs and common positive and negative electrode materials, the present invention has broad application prospects in the field of fast charging batteries.

According to the disclosure and teaching of the above specification, those skilled in the art to which the present invention pertains can also change and modify the above embodiment. Therefore, the present invention is not limited to the above-mentioned specific implementation manners, and any obvious improvement, substitution or modification made by those skilled in the art on the basis of the present invention shall fall within the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims (10)

1. A lithium-ion battery electrolyte, comprising a lithium salt, a non-aqueous organic solvent and an additive, characterized in that: the additive comprises fluoroethylene carbonate, a nitrile compound and a compound having a structure shown in formula I; Wherein, R ₁ , R ₂ , R ₃ , R ₄ , and R ₅ in formula I are independently selected from hydrogen, amido, alkyl or alkoxy with 1 to 3 carbon atoms, and R ₁ , R _2. R ₃ , R ₄ , and R ₅ contain at least one amide group. 2. The lithium-ion battery electrolyte according to claim 1, characterized in that: the mass percentage of the fluoroethylene carbonate in the electrolyte is 1% to 8%; the nitrile compound in the electrolyte The mass percentage content in the electrolyte is 0.05%-5%; the mass percentage content of the compound having the structure shown in formula I in the electrolyte solution is 0.01%-8%. 3. The lithium ion battery electrolyte according to claim 1, wherein the lithium salt is lithium hexafluorophosphate, lithium bisoxalate borate, lithium difluorooxalate borate, lithium bisfluorosulfonyl imide, lithium tetrafluoroborate and At least one of lithium bistrifluoromethanesulfonylimide; the mass percent content of the lithium salt in the electrolyte is 10% to 20%. 4. lithium-ion battery electrolyte according to claim 1, is characterized in that: described non-aqueous organic solvent is selected from ethylene carbonate, propylene carbonate, diethyl carbonate, ethyl methyl carbonate, propyl propionate, Ethyl propionate, methyl propyl carbonate, tetrahydrofuran, dioxane, γ-butyrolactone, ethyl acetate, propyl acetate, methyl acetate, methyl butyrate, ethyl butyrate, propyl butyrate One or more of them; the mass percentage of the non-aqueous organic solvent in the electrolyte is 65% to 85%. 5. lithium-ion battery electrolyte according to claim 1, is characterized in that: described nitrile compound is adiponitrile, succinonitrile, glutaronitrile, pimelonitrile, 2-methylglutaronitrile and 1 , at least one of 3,6-hexanetrinitrile. 6. The lithium-ion battery electrolyte according to claim 1, wherein the compound having the structure shown in formula I is selected from the group consisting of 2,6-dipyridinedicarboxamide, 2-pyridinecarboxamide, 3-pyridine One or more of formamide and 4-pyridinecarboxamide. 7. A lithium ion battery, comprising a positive pole piece, a negative pole piece, a diaphragm arranged between the positive pole piece and the negative pole piece, and an electrolyte, the positive pole piece comprising a positive current collector and a coating The positive electrode membrane covered on the surface of the positive electrode collector, the negative electrode sheet includes the negative electrode collector and the negative electrode membrane coated on the surface of the negative electrode collector, it is characterized in that: the electrolyte is any of claims 1 to 6 One described lithium ion battery electrolyte. 8. The lithium ion battery according to claim 7, characterized in that: the positive electrode membrane includes a positive electrode active material, a conductive agent and a binder, and the compacted density of the positive electrode membrane is ≥4.0g/cm ³ . 9. The lithium-ion battery according to claim 7, characterized in that: the negative electrode membrane includes a negative electrode active material, a conductive agent and a binder, and the surface density of the negative electrode membrane is ≥ 80g/m ² , pressed Solid density ≥ 1.55g/cm ³ . 10. The lithium-ion battery according to claim 7, characterized in that: the charging cut-off voltage of the lithium-ion battery is 4.4-4.7V.