CN111769326A

CN111769326A - Ternary cathode material lithium ion battery electrolyte

Info

Publication number: CN111769326A
Application number: CN202010560663.4A
Authority: CN
Inventors: 俞金萍; 梁大宇; 沈剑
Original assignee: Hefei Guoxuan High Tech Power Energy Co Ltd
Current assignee: Hefei Gotion High Tech Power Energy Co Ltd
Priority date: 2020-06-18
Filing date: 2020-06-18
Publication date: 2020-10-13

Abstract

The invention discloses a ternary cathode material lithium ion battery electrolyte, which comprises the following raw materials: the lithium ion battery comprises an organic solvent, lithium salt, a film forming additive and a functional additive, wherein the functional additive comprises: low-impedance lithium salt, sulfonate organic compounds and borate compounds. According to the invention, the synergistic effect generated by the use of three additives, namely low-impedance lithium salt, sulfonate organic matters and borate compounds, enables the ternary positive material battery to have excellent discharge performance in a low-temperature environment; and the working voltage is 4.4V or above, so that the lithium ion battery has excellent high-temperature cycle performance, and has wide application prospect in a ternary battery system.

Description

Ternary cathode material lithium ion battery electrolyte

Technical Field

The invention relates to the technical field of lithium ion battery electrolyte, in particular to a lithium ion battery electrolyte made of a ternary cathode material.

Background

The lithium ion battery has the characteristics of high working voltage, high energy density, long cycle life, environmental friendliness and the like, and is widely applied to the fields of 3C digital products, electric automobiles and the like. The lithium ion battery is in a discharge increasing trend in the coming years of the application of new energy automobiles, and according to the development and planning of energy-saving and new energy automobile industry, the energy density of the power battery reaches 300Wh/Kg (Wh/Kg) in 2020. Therefore, development of a battery system having a high energy density is imperative.

At present, lithium ion batteries used in the field of new energy automobiles mainly comprise two types, one type is a lithium iron phosphate (LFP) battery with a positive electrode material, and the other type is a ternary material battery. The LFP battery system has the advantages of good cycle performance and relatively reliable safety performance, and has the disadvantages of insufficient energy density and poor low-temperature performance, and particularly, the problem of energy density becomes the main bottleneck of the development of the LFP battery system. The ternary material system is composed of different elements, mainly including NCM and NCA, and can be represented by the general formula LiNi_1-x-yCo_xMn_yO₂And (4) showing. Ni and Mn with lower price are introduced into the NCM material, so that the use of Co is reduced, and the material cost is saved; on the other hand, the NCM can be in the voltage range of 4.35-4.6V, the structure of the NCM is kept stable, and reliable guarantee is provided for the NCM in a power battery system with high voltage and high energy density.

The working voltage of the ternary system which uses a loose battery for supplying power and is represented by Tesla at present is 4.2V, and the development of a ternary power battery with higher voltage and high energy density is a trend of battery technology development and is also an inevitable requirement for new energy gas production development. The electrolyte of the ternary power battery with the voltage of 4.35V or more on the market is still not mature at present, and the main problems are that the specific surface area of the ternary material is large, and the ternary material can generate chemical action with the electrolyte due to the existence of Ni element with stronger oxidizability in a system, so that the performance of the battery is influenced, and particularly the high-low temperature cycle performance and the high-temperature storage performance of the battery cannot be met. Therefore, the development of a ternary material system lithium ion battery electrolyte with a high voltage of more than 4.4V and suitable for the high voltage is urgently needed to meet the practical requirement.

Disclosure of Invention

Based on the technical problems in the background art, the invention provides the ternary cathode material lithium ion battery electrolyte, and the ternary cathode material battery has excellent discharge performance in a low-temperature environment and excellent high-temperature cycle performance under the condition that the working voltage is 4.4V or more and has wide application prospect in a ternary battery system due to the synergistic effect generated by adding functional additives including three additives of low-impedance lithium salt, sulfonate organic matters and borate compounds.

The invention provides a ternary cathode material lithium ion battery electrolyte, which comprises the following raw materials: the lithium ion battery comprises an organic solvent, lithium salt, a film forming additive and a functional additive, wherein the functional additive comprises: low-impedance lithium salt, sulfonate organic compounds and borate compounds.

Preferably, the sulfonate-based organic compound is at least one of 1, 3-propane sultone, 1, 4-butane sultone, ethylene sulfate, vinyl sulfite, methyl methanesulfonate, dimethyl methanesulfonate, diethyl methanesulfonate, diphenyl methanedisulfonate, methylene methanedisulfonate, dimethyl trifluoromethanesulfonate, methyl 2, 4-dimethylbenzenesulfonate and 3-hydroxypropanesulfonic acid.

Preferably, the low impedance lithium salt comprises at least one of lithium difluorophosphate, lithium tetrafluoro oxalate phosphate, lithium difluoro oxalate borate.

Preferably, the borate compound is at least one of trimethyl borate, triethyl borate, triallyl borate, tri-tert-butyl borate, triphenyl borate, tetramethyl borate, tris (2,2, 2-trifluoroethyl) borate, tris (hexafluoroisopropyl) borate, tris (trimethylsilyl) borate, trimethylcyclotriboroxane, tris (pentafluorophenyl) borane.

Preferably, the sulfonate organic matter accounts for 0.05-8% of the total mass of the electrolyte.

Preferably, the sulfonate organic matter accounts for 0.05-2% of the total mass of the electrolyte.

Preferably, the low resistance lithium salt comprises 0.5-10% of the total mass of the electrolyte.

Preferably, the low resistance lithium salt comprises 0.5-2% of the total mass of the electrolyte.

Preferably, the borate compound accounts for 0.05-3% of the total mass of the electrolyte.

Preferably, the content of the functional additive in the electrolyte is less than or equal to 10 percent.

Preferably, the film forming additive is at least one of vinylene carbonate, fluoroethylene carbonate, ethylene carbonate and styrene carbonate.

Preferably, the organic solvent is at least two of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propylene carbonate, ethylene carbonate, methyl propyl carbonate, tetrahydrofuran, dioxane, diethanol diethyl ether, and γ -butyrolactone.

Preferably, the lithium salt is at least one of lithium hexafluorophosphate, lithium bis (fluorosulfonyl) imide, lithium tetrafluoroborate and lithium bis (trifluoromethanesulfonyl) imide.

Preferably, the film forming additive accounts for 0.01-2% of the total mass of the electrolyte.

Preferably, the organic solvent accounts for 65-85% of the total mass of the electrolyte.

Preferably, the lithium salt accounts for 10-15% of the total mass of the electrolyte.

The sum of the weight percentages of the raw materials of the ternary cathode material lithium ion battery electrolyte is 100%.

Has the advantages that:

according to the invention, through the synergistic effect generated by the use of three additives, namely the low-impedance lithium salt, the sulfonate organic matter and the borate compound, the ternary positive material battery has excellent discharge performance in a low-temperature environment, and the working voltage has excellent high-temperature cycle performance under the high-voltage condition of 4.4V or above, so that the ternary positive material battery has a wide application prospect in a ternary battery system.

The functional additive disclosed by the invention is prepared from lithium difluorophosphate, lithium tetrafluoro oxalate phosphate, lithium phosphate, ethylene sulfite, tris (trimethylsilane) borate and other components with good thermal stability and low film-forming impedance, can improve the conductivity of a lithium ion electrolyte at low temperature, and can form a protective film on the surface of an anode under high voltage, so that the protective film can be formed on the surface of the anode, the direct contact reaction and decomposition of the anode and the electrolyte are prevented, the stability of an anode material is improved, and the high-voltage cycle performance of a battery is improved.

In addition, the invention selects a suitable organic solvent which has higher decomposition voltage and better electrochemical stability under high voltage, thereby providing a stable electrochemical environment for the electrical property of the ternary cathode material lithium ion battery with more than 4.4V.

Drawings

Fig. 1 is a plot of the LSV electrochemical window for the electrolyte of comparative example 2.

FIG. 2 is a LSV plot of the electrochemical window of the electrolyte of example 7.

FIG. 3 is a graph showing high temperature cycle performance of the batteries manufactured in comparative examples 1 to 5 and examples 1 to 3, examples 5 to 6, and examples 10 to 12, wherein a is a whole graph, b is comparative examples 1 to 5 and example 1 divided from a, c is examples 1 to 3 divided from a, d is examples 5 to 6 divided from a, and e is examples 10 to 12 divided from a.

Detailed Description

The technical solution of the present invention will be described in detail below with reference to specific examples.

Example 1

The electrolyte of the lithium ion battery with the ternary cathode material comprises the following raw materials in percentage by weight: 2% of fluoroethylene carbonate, 13.5% of lithium hexafluorophosphate, 1% of 1, 3-propane sultone, 1% of lithium difluorophosphate, 0.5% of tris (trimethylsilyl) borate, and the balance of an organic solvent;

wherein the organic solvent is a mixed solvent of ethylene carbonate, propylene carbonate, ethyl methyl carbonate and diethyl carbonate in a mass ratio of 25:5:50: 20.

The preparation method of the ternary cathode material lithium ion battery electrolyte comprises the following steps: in a glove box filled with argon (moisture is less than 10ppm, oxygen content is less than 1ppm), taking an organic solvent, uniformly mixing, then adding fluoroethylene carbonate, 1, 3-propane sultone, lithium difluorophosphate and tri (trimethylsilyl) borate, slowly adding lithium hexafluorophosphate, and stirring until the lithium hexafluorophosphate is completely dissolved.

Comparative example 1

The electrolyte of the lithium ion battery with the ternary cathode material comprises the following raw materials in percentage by weight: 1% of vinylene carbonate, 13.5% of lithium hexafluorophosphate and the balance of an organic solvent; the rest is the same as example 1.

Comparative example 2

The electrolyte of the lithium ion battery with the ternary cathode material comprises the following raw materials in percentage by weight: 1% of vinylene carbonate, 13.5% of lithium hexafluorophosphate, 1% of lithium difluorophosphate and the balance of an organic solvent; the other one is

Example 1.

Comparative example 3

The electrolyte of the lithium ion battery with the ternary cathode material comprises the following raw materials in percentage by weight: 1% of vinylene carbonate, 13.5% of lithium hexafluorophosphate, 1% of lithium difluorophosphate, 1% of 1, 3-propane sultone and the balance of an organic solvent; the rest is the same as example 1.

Comparative example 4

The electrolyte of the lithium ion battery with the ternary cathode material comprises the following raw materials in percentage by weight: 1% of vinylene carbonate, 13.5% of lithium hexafluorophosphate, 1% of lithium difluorophosphate, 0.5% of tris (trimethylsilyl) borate, and the balance being an organic solvent; the rest is the same as example 1.

Comparative example 5

Example 1 was repeated except that "1% of vinylene carbonate" was used in place of "2% of fluoroethylene carbonate".

Example 2

The organic solvent was a mixed solvent of ethylene carbonate, ethyl methyl carbonate and diethyl carbonate in a mass ratio of 30:60:10, and the other examples were the same as example 1.

Example 3

The organic solvent was a mixed solvent of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate and ethyl propionate in a mass ratio of 25:5:55:10:5, and the rest was the same as in example 1.

Example 4

The organic solvent is a mixed solvent of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate and ethyl propionate in a mass ratio of 25:5:55:10: 5;

the procedure of example 1 was repeated except that vinyl sulfite was used in place of 1, 3-propane sultone.

Example 5

the procedure of example 1 was repeated except that 1, 4-butanesultone was used in place of 1, 3-propanesultone.

Example 6

The electrolyte of the lithium ion battery with the ternary cathode material comprises the following raw materials in percentage by weight: 2% fluoroethylene carbonate, 13.5% lithium hexafluorophosphate, 2% 1, 4-butanesultone, 1% lithium difluorophosphate, 0.5% tris (trimethylsilyl) borate, 1% tris (pentafluorophenyl) borane, the balance organic solvent;

wherein the organic solvent is a mixed solvent of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate and ethyl propionate in a mass ratio of 25:5:55:10: 5; the rest is the same as example 1.

Example 7

1, 4-Butanesulfonic acid lactone is 1%, tris (pentafluorophenyl) borane is 0.5%, and the same goes as in example 6.

Example 8

1, 4-Butanesulfonic acid lactone is 1%, the same as in example 6.

Example 9

1, 4-Butanesulfonic acid lactone is 1% and tris (pentafluorophenyl) borane is 2%, the other things being equal to example 6.

Example 10

The electrolyte of the lithium ion battery with the ternary cathode material comprises the following raw materials in percentage by weight: example 6 was repeated except that 2% of fluoroethylene carbonate, 13.5% of lithium hexafluorophosphate, 1% of 1, 4-butanesultone, 0.8% of lithium difluorophosphate, 0.2% of lithium tetrafluoroborate, 0.5% of tris (trimethylsilyl) borate, 1% of tris (pentafluorophenyl) borane and the balance organic solvent were used.

Example 11

The electrolyte of the lithium ion battery with the ternary cathode material comprises the following raw materials in percentage by weight: example 6 was repeated except that 2% of fluoroethylene carbonate, 13.5% of lithium hexafluorophosphate, 1% of 1, 4-butanesultone, 0.5% of lithium difluorophosphate, 0.5% of lithium tetrafluoroborate, 0.5% of tris (trimethylsilyl) borate, 1% of tris (pentafluorophenyl) borane and the balance organic solvent were used.

Example 12

The electrolyte of the lithium ion battery with the ternary cathode material comprises the following raw materials in percentage by weight: example 6 was repeated except that 2% of fluoroethylene carbonate, 13.5% of lithium hexafluorophosphate, 1% of 1, 4-butanesultone, 0.2% of lithium difluorophosphate, 0.8% of lithium tetrafluoroborate, 0.5% of tris (trimethylsilyl) borate, 1% of tris (pentafluorophenyl) borane and the balance organic solvent were used.

Test examples

And (3) taking the electrolytes of examples 1-12 and comparative examples 1-5, respectively injecting the electrolytes into a soft package lithium ion battery with a positive electrode active material of nickel cobalt lithium manganate and a negative electrode active material of graphite, and respectively obtaining the corresponding ternary positive electrode material lithium ion battery after the batteries after injection are subjected to processes of packaging, laying aside, formation, aging, secondary packaging, capacity grading and the like.

The detection method for the lithium ion battery made of the ternary cathode material comprises the following steps:

high temperature 1C/1C cycling experiment: and (3) charging the lithium ion battery with the ternary cathode material to a limiting voltage of 4.4V at 1.0C, changing constant voltage charging until the charging current is less than or equal to a cut-off current, standing for 30min, then discharging at 1.0C until the cut-off voltage is 3.0V, standing for 30min, performing a charge-discharge experiment according to the procedures, and performing cycle for more than 500 weeks.

Low temperature-20 ℃ discharge test: taking the ternary anode material lithium ion battery, changing the charging to 4.4V limiting voltage at room temperature of 25 ℃ by 1C, then changing to constant voltage charging until the charging current is less than or equal to the cutoff current, standing for 5min, then discharging at 1C, and taking the discharging capacity as the initial capacity; charging at 1C to 4.4V, then changing to constant voltage charging until the charging current is less than or equal to the cut-off current, and then placing the battery cell in a thermostat at a low temperature of-20 ℃ in a full-electric state for 16 hours; then, the cell was discharged to 2.8V at 1C, and the discharge capacity was tested. The difference in discharge capacity at low temperature of the battery was calculated and the results are shown in table 1.

TABLE 1 data on low-temperature discharge at-20 ℃ for the batteries prepared in comparative examples 1 to 5 and examples 1 to 12

The data results from table 1 show that:

1) the data results of examples 1 to 3, which compare the capacity development and the capacity retention at low temperatures, show that the electrolyte of the mixed solvent of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate and ethyl propionate, in which the ratio of ethylene carbonate to propylene carbonate to diethyl carbonate to ethyl propionate is 25:5:55:10:5, exerts higher capacity and capacity retention. The addition of the carboxylic ester solvent is obviously helpful to low temperature.

2) Comparative examples 1-5, illustrating fluoroethylene carbonate (FEC), 1, 3-Propane Sultone (PS), lithium difluorophosphate (LiPO)₂F₂) The combined additive with tris (trimethylsilyl) borate (TMSB) is helpful for the low temperature performance of the lithium battery at 4.4V, and the cycle performance at high temperature of 45 ℃ shown in comparison with fig. 3 is poor in the cycle tendency at high temperature; therefore, the electrolyte of example 6 has the best high and low temperature performance in order to achieve a certain high and low temperature performance. Mainly due to-SO in the additive₃the-O-Si-functional group can be decomposed to form a film at the negative electrode, the formed SEI magic has high stability and high conductivity, and the tri (trimethylsilyl) borate additive can stabilize LiFP₆Therefore, the stability of the whole interface of the electrode material is obviously improved, and the electrochemical performance of the electrode material is more stable. Therefore, the additive combination in the electrolyte of example 6 has an interfacial SEI film with good stability and low resistance at ternary materials, especially at high voltage and low temperature.

3) In examples 7 to 9, when the tris (pentafluorophenyl) borane additive was used in a smaller amount, it had better low-temperature discharge performance, and when it was added in an excessive amount, on the one hand, it degraded cycle performance due to excessive decomposition during high-temperature cycling, and on the other hand, an excessive amount of the additive promoted decomposition of the lithium salt and increased interfacial resistance, which decreased the performance of the battery. In examples 10 to 12, by combining lithium difluorophosphate and lithium tetrafluoroborate in different ratios, the results show that the addition of these two lithium salts does not improve the low temperature performance.

FIG. 1 is a LSV plot of the electrochemical window of the electrolyte of comparative example 2, as can be seen in FIG. 1: the electrolyte of comparative example 2 shows an oxidative decomposition peak at 4.2V at the first week of scanning, indicating that the conventional electrolyte undergoes oxidative decomposition of the solvent system after 4.4V.

FIG. 2 is a LSV plot of the electrochemical window of the electrolyte of example 7, as can be seen in FIG. 2: in the electrolyte of example 7, during the first week of scanning, an oxidative decomposition peak is detected at 4.9V, and then an oxidative decomposition peak of a solvent system appears after 5.5V, which indicates that the addition of the functional additive can improve the electrochemical working window of the electrolyte and increase the decomposition voltage of the solvent, mainly because the added combined functional additive decomposes in advance to form a passivation layer on the surface of the positive electrode material, which improves the stability of the electrolyte and the material under high voltage, and has an effect of improving the cycle performance of the battery under high voltage; however, when the amount of the borate compound added is too large, the compound is continuously decomposed, and the by-product substances in the battery system increase, which may adversely affect the battery performance, and the combined amount of the borate additive is preferably limited to 2% or less.

As can be seen from fig. 3:

1) as can be seen from example 1 and comparative examples 1-5, 1% of vinylene carbonate has poor cycle performance in a 4.35V ternary system, and comparative examples 1-3 show that lithium difluorophosphate and 1, 3-propane sultone are beneficial to the cycle performance of a ternary cathode material, and the capacity retention rate is low at low temperature; it can be seen from comparative examples 4-5 that the combination of tris (trimethylsilyl) borate and 1, 3-propane sultone further improves the cycle performance of the battery, and the analysis of the poor cycle performance of the batteries of comparative examples 1-5 is mainly due to insufficient protection of the additive to the interface stability of the positive electrode material, which leads to continuous enhancement of the side reaction with the positive electrode material during the cycle process and deterioration of the cycle life.

2) Comparing examples 1-3, it can be seen from the curves in the figure that the cycle performance of example 3 is improved to some extent, but the cycle performance still generates water jump in the later charge and discharge process; on the other hand, since the solvent contains a carboxylic ester having a low melting point and a low dielectric constant, it has a cracking effect on the cycle performance of the battery.

3) Comparing examples 5-6, it can be seen from the graphs that the high temperature cycle performance of example 6 is further improved, indicating that the high temperature performance deficiencies are compensated for by the synergistic effect of using tris (pentafluorophenyl) borane in combination with a 1, 4-butane sultone mixed additive, in which the-SO is present₃the-O-Si-functional group can be decomposed to form a film at the negative electrode, the formed SEI magic has high stability and high conductivity, the further redox decomposition of the electrolyte can be effectively inhibited, and the tris (trimethylsilyl) borate additive can stabilize the LiFP at the same time₆And HF having a corrosive action is eliminated, thereby inhibiting the dissolution of transition metal ions. Thus, the electrolyte of example 6 has the best cycle life at high temperature and high voltage.

4) Comparing examples 10-12, it can be seen from the graphs in the figures that the use of a combination of lithium difluorophosphate and lithium tetrafluoroborate, it can be seen that the addition of a small amount of lithium tetrafluoroborate has no significant effect on the high temperature electrical properties.

In conclusion, the ternary positive material battery has excellent cycle performance and low-temperature discharge performance under the high potential condition of 4.35V or above through the synergistic effect generated by the use of three additives, namely low-impedance lithium salt, sulfonate organic matters and borate compounds, so that the ternary positive material battery has wide application prospect in a ternary battery system.

The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art should be considered to be within the technical scope of the present invention, and the technical solutions and the inventive concepts thereof according to the present invention should be equivalent or changed within the scope of the present invention.

Claims

1. The electrolyte of the ternary cathode material lithium ion battery is characterized by comprising the following raw materials: the lithium ion battery comprises an organic solvent, lithium salt, a film forming additive and a functional additive, wherein the functional additive comprises: low-impedance lithium salt, sulfonate organic compounds and borate compounds.

2. The electrolyte of the ternary cathode material lithium ion battery according to claim 1, wherein the sulfonate organic compound is at least one of 1, 3-propane sultone, 1, 4-butane sultone, ethylene sulfate, ethylene sulfite, methyl methanesulfonate, dimethyl methanedisulfonate, diethyl methanedisulfonate, diphenyl methanedisulfonate, methylene methanedisulfonate, dimethyl trifluoromethanesulfonate, methyl 2, 4-dimethylbenzenesulfonate, and 3-hydroxypropanesulfonic acid.

3. The ternary positive electrode material lithium ion battery electrolyte of claim 1 or 2, wherein the low resistance lithium salt comprises at least one of lithium difluorophosphate, lithium tetrafluorooxalato phosphate, lithium difluorooxalato borate.

4. The electrolyte for a lithium ion battery as a ternary cathode material according to any one of claims 1 to 3, wherein the borate compound is at least one of trimethyl borate, triethyl borate, triallyl borate, tri-tert-butyl borate, triphenyl borate, tetramethyl borate, tris (2,2, 2-trifluoroethyl) borate, tris (hexafluoroisopropyl) borate, tris (trimethylsilyl) borate, trimethylcyclotriboroxane, tris (pentafluorophenyl) borane.

5. The electrolyte of the ternary cathode material lithium ion battery of any one of claims 1 to 4, wherein sulfonate organic matters account for 0.05 to 8 percent of the total mass of the electrolyte; preferably, the low-resistance lithium salt accounts for 0.5-10% of the total mass of the electrolyte; preferably, the borate compound accounts for 0.05-3% of the total mass of the electrolyte; preferably, the content of the functional additive in the electrolyte is less than or equal to 10 percent.

6. The electrolyte of a ternary cathode material lithium ion battery according to any one of claims 1 to 5, wherein the film forming additive is at least one of vinylene carbonate, fluoroethylene carbonate, ethylene carbonate and styrene carbonate.

7. The electrolyte for a lithium ion battery as a ternary cathode material according to any one of claims 1 to 6, wherein the organic solvent is at least two selected from the group consisting of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, propylene carbonate, ethylene carbonate, propyl methyl carbonate, tetrahydrofuran, dioxane, diethanol diethyl ether, and γ -butyrolactone.

8. The electrolyte of a lithium ion battery with a ternary cathode material according to any one of claims 1 to 7, wherein the lithium salt is at least one of lithium hexafluorophosphate, lithium bis-fluorosulfonylimide, lithium tetrafluoroborate and lithium bis-trifluoromethanesulfonylimide.

9. The electrolyte of a ternary cathode material lithium ion battery according to any one of claims 1 to 8, wherein the film forming additive accounts for 0.01 to 2% of the total mass of the electrolyte.

10. The electrolyte of the ternary cathode material lithium ion battery according to any one of claims 1 to 9, wherein the organic solvent accounts for 65 to 85 percent of the total mass of the electrolyte; preferably, the lithium salt accounts for 10-15% of the total mass of the electrolyte.