CN114824471A - Electrolyte of lithium iron phosphate lithium ion battery with high and low temperature performance - Google Patents

Abstract

本发明利用二苯醚在磷酸铁锂正极表面氧化电位低的特性，对其结构进行设计，以氟原子取代氢原子，防止其分解时产生游离质子，并引入二丁基甲基磷基团作为游离酸吸收剂，得到一种新的全氟二苯醚衍生物。将该全氟二苯醚衍生物作为添加剂加入到电解液中，该添加剂可在磷酸铁锂正极表面氧化形成低聚物保护膜，同时其分解产物可以发挥除酸剂的作用，避免正极侵蚀，提高电池高温性能，有效提高电池的高低温放电性能和循环性能。The invention discloses an electrolyte for a lithium iron phosphate lithium ion battery that takes into account high and low temperature performance, and relates to the technical field of lithium ion batteries. The electrolyte includes lithium salts, high and low temperature additives, other additives, and non-aqueous solvents; The low-temperature additive is a perfluorodiphenyl ether derivative, and its structural formula is:

The invention utilizes the low oxidation potential of diphenyl ether on the surface of the lithium iron phosphate positive electrode, designs its structure, replaces hydrogen atoms with fluorine atoms, prevents free protons from being generated when it decomposes, and introduces dibutyl methyl phosphorus groups as free acids Absorbent, a new perfluorodiphenyl ether derivative was obtained. The perfluorodiphenyl ether derivative is added to the electrolyte as an additive. The additive can be oxidized on the surface of the lithium iron phosphate positive electrode to form an oligomer protective film. Improve the high temperature performance of the battery, effectively improve the high and low temperature discharge performance and cycle performance of the battery.

Description

An electrolyte for lithium iron phosphate lithium ion battery with both high and low temperature performance

technical field

The present invention relates to the technical field of lithium ion batteries, in particular to an electrolyte for a lithium iron phosphate lithium ion battery that takes into account high and low temperature performance.

Background technique

Lithium-ion batteries are widely used in small electronic products, electric vehicles, large-scale energy storage and other fields due to their advantages of high energy density, high operating voltage, good reliability, and low cost. my country has a vast territory with a large north-south span, and the annual temperature difference in some areas can reach 30°C; coupled with the temperature rise caused by the heat generated by the power battery during use, the predictable working and storage temperature of the lithium-ion battery in my country's electric vehicles is about -10°C. ~50℃. Designing a lithium-ion battery system with both high and low temperature performance is necessary and urgent for the application of lithium-ion batteries, especially the popularization and promotion of hybrid and pure electric vehicles.

The influence of low temperature environment on the operation of lithium-ion batteries is mainly reflected in the following aspects: first, during the charging and discharging process at low temperature, the materials, The charge transfer rate will decrease; second, the viscosity of the solvent in the electrolyte increases or solidifies at low temperature, and the ionic conductivity decreases; third, the structural change accompanying the electrode material in the process of delithiation and intercalation at low temperature is hindered, resulting in delithiation and intercalation. / Incomplete intercalation of lithium; Fourth, the chemical and electrochemical stability of the electrolyte composition is reduced at low temperature, and decomposition side reactions occur; For the secondary SEI film, the increase in the film thickness increases the impedance, and at the same time consumes additional active components of the electrolyte. The above factors are interconnected and work together, which ultimately leads to the reduction of the capacity and power of lithium-ion batteries at low temperatures. The impact of high temperature environment on lithium-ion batteries, in addition to vaporizing the volatile components in the electrolyte, causing cell expansion and causing safety problems, also damages the electrodes, resulting in reduced capacity and shortened life. For lithium iron phosphate batteries, the iron dissolution phenomenon will lead to the loss of effective material in the electrode, and even the formation of iron deposits, resulting in the decline of electrical performance and safety performance. The high temperature and free acid in the electrolyte are the main factors causing iron dissolution. One of the ideas to improve the high-temperature performance of batteries is to add additives that can form a stable protective film (CEI) on the surface of the positive electrode. In addition, suppressing the free acid in the electrolyte can also effectively prolong the cycle life of the cell.

In order to solve the above problems faced by lithium-ion batteries in high temperature and low temperature environments, many solutions have been developed from the aspects of battery system design and electrode materials. As the electrolyte is an important part of lithium-ion batteries, the research on low temperature performance mainly focuses on expanding the liquid temperature range of the solvent and adding low temperature additives. Among them, the expansion of the solvent liquid range can be achieved by optimizing the solvent combination, such as adding methyl acetate, ethyl acetate, methyl butyrate, diethyl carbonate, etc. with lower melting points as diluents or co-solvents. The viscosity, ionic conductivity and dielectric constant of the solvent at low temperature must also be considered. Low-temperature additives are mainly used to form SEI films with thin thickness, high ionic conductivity, and stable at low temperature; among them, various fluorine-containing additives including fluorinated long-chain esters and fluorosulfonimides are generally considered to be effective. It is beneficial to the low temperature operation of lithium-ion batteries because it can help to form an SEI film with higher ionic conductivity. For the improvement of high temperature performance, the existing research directions mainly focus on the development of cathode protection additives. However, for lithium iron phosphate cathodes, due to the low working voltage (compared to lithium cobalt oxide and ternary cathodes), there are few suitable oxide film-forming additives.

SUMMARY OF THE INVENTION

Based on the technical problems existing in the background technology, the present invention proposes an electrolyte for a lithium iron phosphate lithium ion battery that takes into account high and low temperature performance. The surface of the iron-lithium positive electrode is oxidized to form an oligomer protective film, and its decomposition products can act as an acid scavenger to avoid corrosion of the positive electrode and improve the high-temperature performance of the battery.

An electrolyte for a lithium iron phosphate lithium ion battery with both high and low temperature properties proposed by the present invention includes lithium salt, high and low temperature additives, other additives, and a non-aqueous solvent; wherein, the high and low temperature additives are perfluorodiphenyl ether derivatives, Its structural formula is shown in formula (I):

The preparation method of above-mentioned perfluorodiphenyl ether derivative is as follows:

1)

2)

3)

Preferably, in terms of mass percentage, it includes: 10-15% of lithium salt, 0.1-8% of high and low temperature additives, 0.01-2% of other additives, and 80-90% of non-aqueous solvent.

Preferably, the other additives are selected from lithium difluorooxalate borate, fluoroethylene carbonate, vinylene carbonate, vinyl sulfate, cyclic sulfate, cyclic sulfite, tris(trimethylsilyl)boron One or more combinations of esters and tris(trimethylsilyl) phosphates.

Preferably, the non-aqueous solvent is selected from ethylene carbonate, propylene carbonate, ethyl methyl carbonate, dimethyl carbonate, butylene carbonate, diethyl carbonate, propyl acetate, ethyl propionate, propyl propionate One or more combinations of esters.

Preferably, the lithium salt is selected from one or more combinations of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis-oxalate borate, lithium bis-fluorosulfonimide, and lithium bis-trifluorosulfonimide.

The present invention also provides a lithium iron phosphate lithium ion battery, comprising the above electrolyte.

The positive electrode material of the above-mentioned lithium iron phosphate lithium ion battery is lithium iron phosphate, and the negative electrode material can be a carbon negative electrode material such as graphite.

Beneficial effects: The present invention utilizes the characteristic of low oxidation potential of diphenyl ether on the surface of the lithium iron phosphate positive electrode, designs its structure, replaces hydrogen atoms with fluorine atoms, prevents the generation of free protons when it decomposes, and also introduces dibutyl methyl phosphorus groups As a free acid absorbent, a new perfluorodiphenyl ether derivative was obtained. The perfluorodiphenyl ether derivative is added to the electrolyte as an additive, and a stable oligomer protective film can be formed on the surface of the lithium iron phosphate positive electrode by oxidation, which plays the role of isolating free acid; and another oxidation product - dibutyl methyl The base phosphorus easily absorbs the free protons introduced by the trace moisture in the electrolyte and avoids the corrosion of the positive electrode, especially in the electrolyte system with lithium hexafluorophosphate as the main lithium salt, it can avoid the formation of hydrofluoric acid, thereby reducing the caused by the lithium iron phosphate material iron Dissolution phenomenon, improve the high temperature performance of the battery. The invention optimizes the ratio and combination of additives in the electrolyte, and effectively improves the capacity retention rate, cycle performance and shelving performance of the lithium iron phosphate-graphite lithium ion battery at high temperature while maintaining the excellent low temperature performance of the battery core.

Detailed ways

An electrolyte for a lithium iron phosphate lithium ion battery with both high and low temperature properties proposed by the present invention includes lithium salt, high and low temperature additives, other additives, and a non-aqueous solvent; wherein, the high and low temperature additives are perfluorodiphenyl ether derivatives, Its structural formula is shown in formula (I):

The preparation method of above-mentioned perfluorodiphenyl ether derivative is as follows:

1)

2)

3)

In the present invention, both the lithium salt and the non-aqueous solvent are commonly used substances in the art, and there is no special requirement. In the following examples, the lithium salt is selected from lithium hexafluorophosphate (LiPF ₆ ); the non-aqueous solvent is composed of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) according to 25:5:55:15 by volume ratio mixing uniformly; high and low temperature additives are compounds represented by formula (I).

Hereinafter, the technical solutions of the present invention will be described in detail through specific embodiments.

Example

The compositions of the electrolytes in Examples 1-8 and Comparative Examples 1-2 are shown in Table 1.

The composition of electrolyte in the embodiment 1-8 of table 1 and comparative example 1-2

Preparation of the above electrolyte: in a glove box filled with argon (oxygen content≤5ppm, water content≤10ppm), the non-aqueous solvents ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), Dimethyl carbonate (DMC) is uniformly mixed according to the volume ratio of 25:5:55:15 with constant stirring. Subsequently, lithium hexafluorophosphate (LiPF ₆ ) was added to the mixed solution and dissolved by stirring well. After the mixed solution returns to room temperature, the high and low temperature additives and other additives are added in sequence, and the mixture is fully stirred and mixed evenly.

An appropriate amount of the electrolyte prepared above was injected into a soft-pack battery with a design capacity of 2.4Ah and vacuum-sealed. The positive electrode of the soft pack battery is lithium iron phosphate, and the negative electrode is natural graphite. The battery can then be prepared through the processes of shelving, chemical formation, aging and capacity separation.

The performances of the batteries prepared in the examples of the present invention and the comparative examples were tested.

1. High and low temperature discharge test: The cell to be tested is subjected to a constant current discharge-constant current and constant voltage charging cycle at 2.4A at room temperature for three weeks, and finally it is charged to 3.65V at 2.4A constant current and constant voltage, and the current is cut off. is 0.12A. Take the average value of the three-week discharge capacity as the reference capacity C ₀ . Put the cell to be tested in a low-temperature liquid-cooling tank and let it stand at -10°C for 5 hours, and then discharge it to 1.8V at a constant current rate of 0.2C ₀ to obtain the low-temperature discharge capacity. Put the cell to be tested in a constant temperature cabinet and let it stand at 55°C for 5 hours, and then discharge it to 1.8V at a constant current rate of 1C ₀ to obtain a high-temperature discharge capacity.

Calculation method of high and low temperature discharge capacity retention rate: (constant current discharge capacity at target temperature)/(constant capacity capacity at normal temperature)×100%.

2. Cycle performance test: Place the formed lithium-ion battery in the environment of -10°C and 55°C, respectively, at 0.2C ₀ /0.2C ₀ , 1C ₀ /1C ₀ constant current and constant voltage charge/cross-current discharge to the cut-off voltage. High and low temperature cycle. Calculate the weekly capacity retention rate and record the current cycle number when the capacity retention rate is below 80%.

Calculation method of cycle capacity retention rate: (cycle discharge capacity per week)/(average discharge capacity of 5 weeks before cycle)×100%.

The results are shown in Table 2-3 respectively.

Table 2 Discharge capacity retention rate of electrolyte at high temperature and low temperature

It can be seen from Tables 2-4 that in Examples 1-7 with the addition of the perfluorodiphenyl ether derivative additive of formula (I), the high temperature discharge capacity retention rate of the battery cell is improved. Comparing Example 1 and Comparative Example 2, the high and low temperature properties of both are similar, indicating that the additive represented by formula (I) can replace VC, thereby reducing cost and reducing dependence on VC. Comparing Example 1, Example 2, Example 3 and Example 8, it can be found that when the concentration of the additive shown in structural formula (I) exceeds 1%, the high-temperature and low-temperature discharge performance declines instead. The reason is that the thickness of the positive electrode protective film is too large, Internal resistance rises. Comparing Example 1 and Example 6, the low temperature performance is somewhat reduced after replacing the LiDFOB+FEC combination with VC, while the additive represented by formula (I) can still ensure the high temperature discharge performance. Comparing Example 1 and Example 4, the additive represented by structural formula I does not affect the effect of FEC on improving high temperature performance.

Table 3 Cycling performance of electrolyte at high temperature and low temperature

Compared with Comparative Example 1-2, the high and low temperature performance in Example 1 is better than Comparative Example 1-2, indicating that the perfluorodiphenyl ether derivative additive of the present invention can effectively improve the high and low temperature of lithium iron phosphate-graphite battery. cycle life.

The above description is only a preferred embodiment of the present invention, but the protection scope of the present invention is not limited to this. The equivalent replacement or change of the inventive concept thereof shall be included within the protection scope of the present invention.

Claims (6)

1. an electrolyte of a lithium iron phosphate lithium ion battery taking into account high and low temperature performance, is characterized in that, comprises lithium salt, high and low temperature additive, other additives, non-aqueous solvent; Wherein, high and low temperature additive is derived from perfluorodiphenyl ether compound, its structural formula is shown in formula (I):

2. the electrolyte of the lithium iron phosphate lithium ion battery taking into account high and low temperature performance according to claim 1, is characterized in that, by mass percentage, comprises: lithium salt 10-15%, high and low temperature additives 0.1-8%, Other additives 0.01-2%, non-aqueous solvent 80-90%. 3. the electrolyte of the lithium iron phosphate lithium ion battery taking into account high and low temperature performance according to claim 1 and 2, it is characterized in that, described other additive is selected from difluorooxalate lithium borate, fluoroethylene carbonate, subcarbonate One or more combinations of vinyl ester, vinyl sulfate, cyclic sulfate, cyclic sulfite, tris(trimethylsilyl) borate, and tris(trimethylsilyl) phosphate. 4. the electrolyte of the lithium iron phosphate lithium ion battery taking into account high and low temperature performance according to claim 1 and 2, it is characterized in that, described non-aqueous solvent is selected from ethylene carbonate, propylene carbonate, methyl ethyl carbonate, One or more combinations of dimethyl carbonate, butylene carbonate, diethyl carbonate, propyl acetate, ethyl propionate, and propyl propionate. 5. the electrolyte of the lithium iron phosphate lithium ion battery taking into account high and low temperature performance according to claim 1 and 2, it is characterized in that, described lithium salt is selected from lithium hexafluorophosphate, lithium tetrafluoroborate, bis-oxalate lithium borate, bis-fluorine One or more combinations of lithium sulfonimide and lithium bistrifluorosulfonimide. 6. A lithium iron phosphate lithium-ion battery, characterized in that it comprises the electrolyte according to any one of claims 1-5.