CN113764733B

CN113764733B - Electrolyte and lithium ion battery

Info

Publication number: CN113764733B
Application number: CN202010491107.6A
Authority: CN
Inventors: 陶蒙; 王圣; 郭典达; 刘刚
Original assignee: BYD Co Ltd
Current assignee: BYD Co Ltd
Priority date: 2020-06-02
Filing date: 2020-06-02
Publication date: 2023-12-12
Anticipated expiration: 2040-06-02
Also published as: CN113764733A

Abstract

The application provides an electrolyte, which comprises an organic solvent, lithium salt and a flame retardant additive, wherein the flame retardant additive has a structure shown in a formula (1),formula (1), wherein R ₁ 、R ₂ Independently selected from R ₄ ‑N‑R ₃ One of pyrrolyl and piperidyl, R ₃ 、R ₄ Independently selected from one of hydrogen, alkyl, haloalkyl, aryl, haloaryl, ester group, alkoxy and amide. The electrolyte is introduced with the flame retardant additive with a spiro structure, so that the cycle performance of the battery is not affected while the flame retardant performance of the electrolyte is improved.

Description

Electrolyte and lithium ion battery

Technical Field

The application relates to the field of electrolyte for lithium ion batteries, in particular to electrolyte and a lithium ion battery.

Background

The lithium ion battery has the advantages of high energy density, good cycle performance, no memory effect and the like, and is widely applied to the fields of electronics, automobiles and the like. The existing lithium ion battery comprises a positive electrode, a negative electrode, a diaphragm and electrolyte, wherein the electrolyte contains an organic solvent, and the organic solvent has the defects of inflammability and volatility as known in the art, so that the battery also has potential safety hazards to a certain extent, and particularly under abusive conditions (such as high temperature, overcharging, overdischarge, short circuit, vibration, extrusion, impact and the like), the lithium ion battery is more prone to smoke, fire and even explosion, and the like, and as a result, the safety accidents related to thermal runaway caused by the abuse of the lithium ion battery can occur every year. In order to improve the safety performance of the lithium ion battery and reduce the risk of combustion and explosion caused by thermal runaway of the lithium ion battery, the improvement of the electrolyte is particularly important. In the prior reported flame-retardant electrolyte, phosphate additives are mostly used for improving the flame-retardant performance of the electrolyte, such as common trimethyl phosphate (TMP), triethyl phosphate (TEP) and triphenyl phosphate (TPP), however, the phosphate additives have larger viscosity and poor compatibility with electrode materials (especially carbon-based cathodes), and during the charging process, co-intercalation of solvents can occur, so that the cathode active materials are peeled off, and the flame-retardant capability of the electrolyte is improved and the cycle life of the battery is greatly shortened.

Disclosure of Invention

In order to solve the technical problems that the electrolyte of the lithium ion battery is inflammable, and the existing electrolyte with flame retardant property is used for the battery, so that the cycle performance of the battery is reduced, the application provides the electrolyte and the lithium ion battery.

In a first aspect, the present application provides an electrolyte comprising an organic solvent, a lithium salt and a flame retardant additive, characterized in that the flame retardant additive has a structure represented by formula (1),

(1),

wherein R is ₁ 、R ₂ Independently selected from R ₄ -N-R ₃ One of pyrrolyl and piperidyl, R ₃ 、R ₄ Independently selected from one of hydrogen, alkyl, haloalkyl, aryl, haloaryl, ester group, alkoxy and amide.

Compared with the electrolyte in the prior art, the electrolyte disclosed by the application contains the flame retardant additive shown in the formula (1), the additive contains phosphorus element, and under the heating condition, the additive is gasified to release phosphorus-containing free radicals, and the phosphorus-containing free radicals are combined with hydrogen free radicals, so that the chain reaction of the free radicals is blocked, the combustion process of the electrolyte cannot be carried out or is difficult to carry out, and the flame retardant property of the electrolyte is further improved. Compared with the existing phosphate additives, the additive in the electrolyte also contains nitrogen element, and the nitrogen element has a flame retardant effect, so that the safety performance of the electrolyte can be further improved; moreover, as the additive of the electrolyte has a spiro structure, the molecule is larger, and the co-intercalation with lithium ions is not easy to occur in the battery cycle process, thereby inhibiting the peeling of the graphite negative electrode, greatly improving the compatibility of the electrolyte and the battery negative electrode, and improving the cycle performance and the service life of the battery.

In a second aspect, the application provides a lithium ion battery comprising the electrolyte as described above.

The electrolyte of the lithium ion battery contains the flame retardant additive shown in the formula (1), and the additive contains higher content of phosphorus element and nitrogen element, so that the electrolyte has better flame retardant effect; moreover, since the additive has a large molecular structure, when a material capable of intercalating lithium ions such as graphite is used as the negative electrode active material, co-intercalation with lithium ions does not occur, and thus exfoliation of the negative electrode active material is not caused, thereby contributing to improvement of cycle performance of the battery. Therefore, the lithium ion battery provided by the application is beneficial to improving the cycle performance of the battery while improving the flame retardant performance of the battery.

Detailed Description

In order to make the technical problems, technical schemes and beneficial effects solved by the application more clear, the application is further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the application.

In a first aspect, the present application provides an electrolyte comprising an organic solvent, a lithium salt, and a flame retardant additive, wherein the flame retardant additive has a structure represented by formula (1),

(1),

wherein R is ₁ 、R ₂ Independently selected from R ₄ -N-R ₃ One of pyrrolyl and piperidyl, R ₃ 、R ₄ Independently selected from hydrogen, alkyl, haloalkyl, aryl, haloaryl, ester, and alkylAn oxy group and an amide group.

The flame retardant additive contained in the electrolyte is different from phosphate additives in the prior art, and the electrolyte additive has a good flame retardant effect and does not influence the cycle performance of the battery. The flame retardant additive in the electrolyte has a structure shown in a formula (1), contains phosphorus element and nitrogen element, and can react to generate NO and NH during high-temperature pyrolysis ₃ The non-combustible gas such as oxygen in the dilution air is acted, the phosphorus element can release phosphorus-containing free radicals, and the phosphorus-containing free radicals can be combined with hydrogen free radicals generated by the electrolyte, so that chain reaction of the free radicals can be blocked, the combustion process of the electrolyte can not be performed or is difficult to perform, and meanwhile, the phosphorus element and the nitrogen element are contained, so that the oxygen and the hydrogen free radicals necessary for the combustion of the electrolyte can be diluted and inhibited, the flame retardant property of the additive is greatly improved, and the safety performance of the electrolyte is further improved. In addition, compared with the phosphate additives in the prior art, the flame retardant additive has a larger molecular structure, so that the phenomenon of stripping of the anode material caused by co-intercalation of the flame retardant additive and lithium ions in the battery cycle can not occur, and the cycle performance of the battery can be improved.

Further, the alkyl group is selected from one of a chain alkyl group or a cyclic alkyl group, and the haloalkyl group is selected from one of a halogen-substituted chain alkyl group or a cyclic alkyl group.

Preferably, the alkyl is selected from the group consisting of chain alkyl groups and the haloalkyl is selected from the group consisting of halogenated chain alkyl groups.

Compared with the cyclic alkyl, the chain alkyl has higher cathode stability, namely the phenomenon of co-intercalation with lithium ions in the cathode can not occur in the cycle process of the battery, thereby the cathode active material can not be peeled off, and the cycle performance of the battery is improved.

Further, the alkyl group is selected from C ₁ -C ₁₀ Is selected from C ₁ -C ₁₀ Is selected from the group consisting of C ₁ -C ₁₀ Aryl, halogenated aryl is selected from C ₁ -C ₁₀ Is halogenated of (2)Aryl groups. .

The number of carbon atoms in the substituent is not more than 10, namely the substituent in the formula (1) is substituted by a small group, the phosphorus element content in the additive molecule can be further improved by selecting the small group for substitution, and the higher the phosphorus element content is, the better the flame retardant effect of the additive is.

Further, the mass content of the phosphorus element in the flame retardant additive is not less than 10%, and the mass content of the nitrogen element is not less than 5%.

The phosphorus element in the flame retardant additive is helpful for combining hydrogen free radical generated by electrolyte, the nitrogen element in the flame retardant additive is helpful for consuming and diluting oxygen in air, and a great deal of experimental researches by a person skilled in the art show that when the mass content of the phosphorus element in the flame retardant additive is not less than 10% and the mass content of the nitrogen element is not less than 5%, the flame retardant additive has better flame retardant property.

Further, R ₃ 、R ₄ Each independently selected from at least one of a haloalkyl group and a haloaryl group, and the mass content of halogen in the flame retardant additive is not less than 10%.

The flame retardant additive contains halogen substituted alkyl and/or aryl, so that the flame retardant performance of the flame retardant additive can be greatly improved, because the halogen also has flame retardant effect, the halogen can generate hydrogen halide during thermal decomposition, and can capture hydrogen free radicals and hydroxyl free radicals generated by the decomposition of the electrolyte, so that the concentration of the free radicals in the electrolyte is greatly reduced, the free radical chain reaction of the combustion of the electrolyte can be slowed down or stopped, and the safety performance of the electrolyte is further improved. Thus, R in the flame retardant additive ₃ 、R ₄ Preferably at least one selected from the group consisting of haloalkyl and haloaryl. Moreover, a great deal of experimental study by the person skilled in the art also shows that when the halogen content in the flame retardant additive is not less than 10%, a better flame retardant effect can be achieved, and the high temperature stability of the electrolyte is greatly improved.

Further, halogen is selected from bromine.

The halogen in the flame retardant additive is selected from bromine elements according to the difficulty of chemical bond fracture between carbon atoms and halogen atoms, because the C-Br bond is easier to fracture, hydrogen bromide is easier to generate and is used for capturing the free radicals, and the combustion reaction of the electrolyte is inhibited.

Further, the mass ratio of the flame retardant additive is 20-30% based on the total mass of the electrolyte.

The content of the flame retardant additive is in the range, so that the electrolyte has better flame retardant property, and the dissolution of lithium salt in the electrolyte is not influenced, so that the ion conductivity of the electrolyte is not influenced, namely the transmission of lithium ions in the electrolyte is not influenced, and the battery has better electrochemical property.

Further, the flame retardant additive is one or more of the formulas (2) to (8),

(2),>(3),

(4),>(5),

(6),>(7),

(8),>formula (9).

Further, the electrolyte also contains a film forming additive, wherein the film forming additive is selected from one or more of FEC (fluoroethylene carbonate), DFEC (bifluorinated ethylene carbonate) and DTD (ethylene sulfate), and the mass ratio of the film forming additive in the electrolyte is 1-5%.

Further, the organic solvent is selected from one or more of ethylene carbonate, methyl ethyl carbonate, dimethyl carbonate, diethyl carbonate, methyl propionate and ethyl propionate.

Further, the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium bis (fluorosulfonyl) imide and lithium bis (trifluorosulfonyl) imide, and the concentration of the lithium salt in the electrolyte is 1.0-1.5mol/L.

The electrolyte contains a flame retardant additive shown in the formula (1), and the flame retardant additive has higher content of phosphorus element and nitrogen element, so that the flame retardant additive has better flame retardant property; the flame retardant additive has a larger molecular structure, so that the phenomenon of co-intercalation of the additive and lithium ions does not occur, the peeling of the anode active material is avoided, and the cycle performance of the battery is greatly improved.

Further, the positive electrode plate and the negative electrode plate are also included.

Wherein the positive plate comprises a positive electrode active material selected from LiCoO ₂ 、LiNi _0.5 Mn _1.5 O ₄ 、LiNi _0.33 Co _0.33 Mn _0.33 O ₂ One or more of the following; the negative electrode sheet comprises a negative electrode active material, and the negative electrode active material is one or more selected from carbon materials, graphite and MCMB.

The present application is further illustrated in detail by the following examples, which are provided for the purpose of illustration and explanation only and are not intended to be limiting.

Example 1

Preparation of electrolyte:

40 mL Ethylene Carbonate (EC), 60 mL carbonMethyl ethyl acetate (EMC) was mixed to a mixed solvent, and 7.6. 7.6 g lithium hexafluorophosphate (LiPF) was added to the mixed solvent ₆ ) And 9.35. 9.35 g lithium bis (fluorosulfonyl) imide (LiFSI), wherein the concentrations of the two are 0.5 mol/L and 0.5 mol/L, respectively. Then 32.5g of 3, 9-bis (N-cyclohexylamino) -2,4,8, 10-tetraoxy-3, 9-dioxo-3, 9-diphospho spiro [5,5] as a flame retardant additive were added thereto]Undecane, formula (2) in the specification, and the flame retardant additive accounts for 20% of the electrolyte.

Preparation of the battery:

the electrolyte, the positive plate, the negative plate and the diaphragm form a battery, wherein the positive plate contains positive active material LiCoO ₂ The negative electrode active material contained in the negative electrode plate is graphite.

Example 2

Unlike example 1, the flame retardant additive was 3, 9-bis (N-cyclopentyl) -2,4,8, 10-tetraoxy-3, 9-dioxo-3, 9-diphosphaspiro [5,5] undecane, i.e. formula (3) in the specification, the flame retardant additive accounted for 25% of the electrolyte mass; the battery preparation method was the same as in example 1.

Example 3

Unlike example 1, the flame retardant additive was 3, 9-bis (piperidinyl) -2,4,8, 10-tetraoxy-3, 9-dioxo-3, 9-diphospho spiro [5,5] undecane, formula (4) in the specification, the flame retardant additive accounting for 30% of the electrolyte mass. The method comprises the steps of carrying out a first treatment on the surface of the The battery preparation method was the same as in example 1.

Example 4

Unlike example 1, the flame retardant additive is 3, 9-bis (pyrrolidinyl) -2,4,8, 10-tetraoxy-3, 9-dioxo-3, 9-diphosphaspiro [5,5] undecane, i.e., formula (5) of the specification; the battery preparation method was the same as in example 1.

Example 5

Unlike example 1, the flame retardant additive was 3, 9-bis (N, N-diethylamino) -2,4,8, 10-tetraoxy-3, 9-dioxo-3, 9-diphosphaspiro [5,5] undecane, i.e., formula (6) of the specification; the battery preparation method was the same as in example 1.

Example 6

Unlike example 1, the flame retardant additive was 3, 9-bis (N-p-chlorophenyl) -2,4,8, 10-tetraoxy-3, 9-dioxo-3, 9-diphospho spiro [5,5] undecane, i.e., formula (7); the battery preparation method was the same as in example 1.

Example 7

Unlike example 1, the flame retardant additive was 3, 9-bis (N-chloromethyl) -2,4,8, 10-tetraoxy-3, 9-dioxo-3, 9-diphosphaspiro [5,5] undecane, i.e., formula (8) of the specification; the battery preparation method was the same as in example 1.

Example 8

Unlike example 1, the flame retardant additive was 3, 9-bis (N-chloromethyl) -2,4,8, 10-tetraoxy-3, 9-dioxo-3, 9-diphospho spiro [5,5] undecane, formula (8) in the specification, with the addition of 3.6. 3.6 g bis-fluoroethylene carbonate as a film forming additive, and the battery was prepared in the same manner as in example 1.

Example 9

Unlike example 1, the flame retardant additive was 3, 9-bis (N-bromomethyl) -2,4,8, 10-tetraoxy-3, 9-dioxo-3, 9-diphospho spiro [5,5] undecane, formula (9) in the specification, with 3.6. 3.6 g bis fluoroethylene carbonate added as a film forming additive, and the battery was prepared in the same manner as in example 1.

Example 10

Unlike example 1, the flame retardant additive was 3, 9-bis (N-biphenyl) -2,4,8, 10-tetraoxy-3, 9-dioxo-3, 9-diphospho spiro [5,5] undecane, the flame retardant additive accounting for 20% of the electrolyte mass; the battery preparation method was the same as in example 1.

Example 11

Unlike example 1, the flame retardant additive was 3, 9-bis (N-p-chlorobiphenyl) -2,4,8, 10-tetraoxy-3, 9-dioxo-3, 9-diphospho spiro [5,5] undecane, the flame retardant additive accounting for 20% of the electrolyte mass; the battery preparation method was the same as in example 1.

Example 12

Unlike example 1, the flame retardant additive accounts for 5% of the electrolyte mass; the battery preparation method was the same as in example 1.

Comparative example 1

Unlike example 1, the battery was prepared in the same manner as in example 1 except that the flame retardant additive was triphenyl phosphate of 30 g.

Comparative example 2

Unlike example 1, which was a flame retardant additive of trimethyl phosphate, 30 g, the battery was prepared in the same manner as example 1.

Comparative example 3

Unlike example 1, the battery was prepared in the same manner as example 1 without adding a flame retardant additive.

Self-extinguishing time test

The self-extinguishing time is the time from when the ignition device is removed to when the flame is automatically extinguished, and is used for comparing the flame retardant performance of different flame retardant electrolytes by taking the self-extinguishing time of the electrolyte of unit mass as a standard. And (3) dripping the prepared electrolyte on glass fiber filter paper, weighing, igniting, recording the time from ignition to extinction of the electrolyte, and dividing the time by the mass of the electrolyte on the glass fiber filter paper to obtain the self-extinction time.

The electrolytes of examples 1 to 12 and comparative examples 1 to 3 were subjected to self-extinguishing time test, each of which was performed 5 times, and the average value was taken, and the experimental test results are shown in table 1.

TABLE 1

Project	P element content (%)	N element content (%)	Cl element content (%)	Self-extinguishing time (s/g)
					Example 1	14.6	6.6	0	38
Example 2	15.7	7.1	0	31
					Example 3	15.7	7.1	0	31
Example 4	16.9	7.7	0	28
					Example 5	16.7	7.6	0	28
Example 6	12.9	5.9	14.8	25
					Example 7	17.4	7.9	19.9	16
Example 8	17.4	7.9	19.9	16
					Example 9	14.1	6.4	18.2	14
Example 10	10.0	4.9	0	50
					Example 11	9.8	4.4	11.1	55
Example 12	14.6	6.6	0	45
					Comparative example 1	9.5	0	0	45
Comparative example 2	22.1	0	0	25
					Comparative example 3	0	0	0	80

Capacity retention test

After the battery was packaged and left to stand overnight, the batteries of examples 1 to 12 and comparative examples 1 to 3 were subjected to a test for battery capacity retention in an incubator at 25C, the batteries were cycled between 0.005V and 1V 100 times at a current of 0.2C (0.3 mA), and the discharge capacity at the 100 th cycle divided by the maximum discharge capacity was a percentage of battery capacity retention (%), wherein 20 batteries were tested, and the results were averaged, and the results are shown in table 2.

TABLE 2

Project	0.2C Capacity retention (%)
		Example 1	67.8
Example 2	77.4
		Example 3	77.0
Example 4	85.0
		Example 5	87.5
Example 6	87.7
		Example 7	90.4
Example 8	91.7
		Example 9	92.0
Example 10	77.4
		Example 11	77.5
Example 12	78.1
		Comparative example 1	76.4
Comparative example 2	14.3
		Comparative example 3	85.4

As can be seen from Table 1, the electrolyte containing the flame retardant additive of the structure shown in the formula (1) of the present application has excellent flame retardant properties, especially, in example 9, the flame retardant additive contains 3, 9-bis (N-bromomethyl) -2,4,8, 10-tetraoxy-3, 9-dioxo-3, 9-diphosphocyclo [5,5] undecane, the self-extinguishing time is 14 s/g at an addition amount of 20%, and has a smaller self-extinguishing time compared with the flame retardant additives in other examples, mainly because the 3, 9-bis (N-bromomethyl) -2,4,8, 10-tetraoxy-3, 9-diphosphocyclo [5,5] undecane has a higher content of phosphorus element and nitrogen element, and also contains a higher content of halogen element, the flame retardant properties of the additive can be further improved because the halogen thermally generated halogen can decompose the free radicals generated in the combustion of the electrolyte, so that the chain reaction of the flame retardant additive can be further improved or terminated; in particular, the additive contains bromine element, so that the flame retardant effect is better.

Furthermore, the electrolyte containing the flame retardant additive of the structure represented by the formula (1) of the present application has excellent battery cycle performance. As can be seen from table 2, the electrolyte containing the flame retardant additive of the structure shown in formula (1) also has excellent battery cycle performance as compared with the trimethyl phosphate and triphenyl phosphate flame retardant additives of the prior art.

By combining the table 1 and the table 2, the flame retardant additive in the application has certain flame retardant property, and simultaneously has a larger molecular structure, so that the co-intercalation of flame retardant additive molecules and lithium ions is not easy to occur, thereby ensuring the compatibility and stability with negative electrode active materials such as graphite and the like; in comparison, the trimethyl phosphate flame retardant additive of the prior art is embedded into the graphite negative electrode during the battery cycle, thereby causing exfoliation of the graphite negative electrode, so that the battery cycle decays faster (table 2, comparative example 2). For triphenyl phosphate with the same larger molecular structure, although the influence on the cycle performance of the battery is small, the flame retardant effect of the triphenyl phosphate is not good. The electrolyte provided by the application not only has good flame retardant property, but also has stability to negative electrode active materials such as graphite and the like, and improves the cycle performance of the battery to a certain extent.

Claims

1. An electrolyte comprises an organic solvent, lithium salt and a flame retardant additive, and is characterized in that the flame retardant additive is one or more of formulas (7) to (9),

in the flame retardant additive, the mass content of halogen is not less than 10%.

2. The electrolyte according to claim 1, wherein the mass content of phosphorus element in the flame retardant additive is not less than 10% and the mass content of nitrogen element is not less than 5%.

3. The electrolyte of claim 1 wherein the flame retardant additive comprises 20 to 30% by mass based on the total mass of the electrolyte.

4. The electrolyte of claim 1, further comprising a film forming additive selected from one or more of FEC (fluoroethylene carbonate), DFEC (bis-fluoroethylene carbonate), DTD (ethylene sulfate).

5. A lithium ion battery comprising the electrolyte of any one of claims 1-4.