Electrolyte additive adaptive to lithium iron manganese phosphate battery and electrolyte

Abstract

The invention discloses an electrolyte additive adaptive to a lithium manganese iron phosphate battery and an electrolyte, the additive amount of the electrolyte additive is 0.1-3% of the total mass of the electrolyte, and the electrolyte additive contains two benzene rings and two sulfenyl groups; two benzene rings of the electrolyte additive maintain a hexagonal plane structure, and each benzene ring is attached with six hydrogen atoms; two benzene rings of the electrolyte additive are structural units connected through a sulfur atom; the aromatic molecule part of phenyl in the electrolyte additive can be subjected to a polymerization reaction with the surface of a positive electrode to form a positive electrode CEI film, and sulfenyl in the electrolyte additive has a relatively high HOMO energy level and can be reduced to form an SEI film at a negative electrode prior to an electrolyte solvent; the sulfur group in the electrolyte additive compound is easy to accept electrons, and forms a lithium-oxygen-sulfur compound on the surface of a negative electrode, so that the electrolyte decomposition of a negative electrode interface is reduced, the dissolution of metal Mn in a positive electrode is inhibited, and the high-temperature long-cycle stability of the battery is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of electrolytes, and specifically provides an electrolyte additive and an electrolyte suitable for lithium iron manganese phosphate batteries. Background Art

[0002] Currently, mainstream lithium-ion energy storage and power batteries mostly use lithium nickel cobalt manganese oxide or lithium iron phosphate as the cathode material. The former has relatively high cathode energy density and voltage platform, but poor safety and high manufacturing cost. The latter is favored for its excellent stability and safety performance, as well as the good structural stability during charge and discharge brought by the olivine structure. However, its voltage platform is relatively low and its performance in low-temperature environments is relatively weak.

[0003] Compared with the above two widely used cathode materials, lithium iron manganese phosphate exhibits unique advantages. Although its theoretical capacity is the same as that of lithium iron phosphate, both being 170 mAh / g, in terms of the electrode potential of Li+ / Li, lithium iron manganese phosphate is as high as 4.1 V, significantly higher than 3.4 V of lithium iron phosphate, and it is exactly within the stable electrochemical window of the organic electrolyte system. This high potential of 4.1 V endows lithium iron manganese phosphate with potential high energy density characteristics, which is its greatest competitive advantage compared with lithium iron phosphate.

[0004] If the actual capacity of lithium iron manganese phosphate is developed to be the same as that of lithium iron phosphate, the energy density of lithium iron manganese phosphate will be increased by 15%, and compared with lithium manganate with a similar voltage, its mass energy density can be increased by more than 25%. In addition, lithium iron manganese phosphate has better performance under low-temperature conditions, and its low-temperature capacity retention rate can almost rival or even exceed some ternary materials. At the same time, lithium iron manganese phosphate has low raw material costs, excellent environmental protection performance, and broad market prospects;

[0005] Lithium iron manganese phosphate has low raw material costs, excellent environmental protection performance, and broad market prospects. However, it should be noted that during high-temperature storage and cycling, Mn inside lithium iron manganese phosphate 3+ will dissolve out, resulting in a disproportionation reaction on the surface of the cathode material to generate Mn 2+ and Mn 4+ . Among them, the Mn metal that migrates to the surface of the negative electrode and is reduced and deposited will damage the SEI film, thereby triggering a series of problems such as continuous thickening of the SEI film, increased loss of active lithium, increased electrode resistance, and gas generation at the negative electrode, ultimately resulting in continuous attenuation of the battery capacity. Summary of the Invention

[0006] The purpose of the present invention is to provide an electrolyte additive and an electrolyte suitable for lithium iron manganese phosphate batteries to solve the problems raised in the above background art.

[0007] To achieve the above object, the present invention provides the following technical solution: An electrolyte additive adapted to a lithium iron manganese phosphate battery, comprising:

[0008] The addition amount of the electrolyte additive is 0.1-3% of the total mass of the electrolyte, and the electrolyte additive contains two benzene rings and two sulfhydryl groups;

[0009] The two benzene rings of the electrolyte additive maintain a hexagonal planar structure, and each benzene ring is attached with six hydrogen atoms;

[0010] The two benzene rings of the electrolyte additive are structural units connected by a sulfur atom;

[0011] The sulfur atom in the electrolyte additive serves as a bridging element and is located between the two benzene rings, forming a sulfide group;

[0012] The structural formula of the electrolyte additive is:

[0013]

[0014] An electrolyte containing the above electrolyte additive adapted to a lithium iron manganese phosphate battery, comprising the above electrolyte additive, a lithium salt electrolyte, a non-aqueous organic solvent, and an auxiliary additive.

[0015] Among them, the addition concentration of the lithium salt electrolyte is 1.2 mol / L.

[0016] Among them, the lithium salt electrolyte is one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluorodioxalate, lithium bis(fluorosulfonyl)imide salt, and lithium dioxalate borate.

[0017] Among them, the non-aqueous organic solvent is a mixture composed of any two of ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and propylene carbonate.

[0018] Among them, the addition amount of the non-aqueous organic solvent is 90-95% of the total mass of the electrolyte.

[0019] Among them, the auxiliary additive is one or more of vinylene carbonate, ethylene vinylene carbonate, and propylene sulfite.

[0020] Among them, the addition amount of the auxiliary additive is 1-5% of the total mass of the electrolyte.

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] 1. The addition amount of the electrolyte additive of the present invention is 0.1-3% of the total mass of the electrolyte. Only a minor adjustment to the existing electrolyte system is required to adapt to the lithium iron manganese phosphate cathode material, and the high-temperature storage and high-temperature cycle life of the battery are greatly improved, which is very conducive to large-scale production;

[0023] 2. The aromatic molecular part of the phenyl group in the electrolyte additive of the present invention can undergo a polymerization reaction with the surface of the cathode to form a cathode CEI film. The sulfhydryl group in the electrolyte additive has a relatively high HOMO energy level and can preferentially reduce on the anode surface to form an SEI film prior to the electrolyte solvent; the sulfhydryl group in this electrolyte additive compound easily accepts electrons to form a lithium-oxygen-sulfur compound on the anode surface, reducing the decomposition of the electrolyte at the anode interface and inhibiting the dissolution of metal Mn in the cathode, thereby increasing the high-temperature long-cycle stability of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Schematic diagram of the linear sweep voltammetry curves of electrolyte 1 and electrolyte 2 of the present invention;

[0025] Figure 2 Schematic diagram of the reaction mechanism of the electrolyte additive of the present invention;

[0026] Figure 3 Schematic diagram of the performance comparison of the three groups of battery cycle tests of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0028] Example 1

[0029] Preparation of electrolyte:

[0030] In a glove box with an argon atmosphere and environmental indicators of H2O ≤ 0.1 ppm and O2 ≤ 0.1 ppm, the electrolyte uses 92% by weight of a non-aqueous organic solvent (ethylene carbonate and diethyl carbonate, with a mass ratio of 1:1), mixes in 2% by weight of a lithium salt electrolyte (lithium hexafluorophosphate) with a concentration of 1.2 mol / L, and adds 1% by weight of the electrolyte additive and 5% by weight of an auxiliary additive (vinylene carbonate) to obtain electrolyte 1.

[0031] Example 2

[0032] Preparation of electrolyte:

[0033] In a glove box under an argon atmosphere with environmental indicators of H2O ≤ 0.1 ppm and O2 ≤ 0.1 ppm, the electrolyte is prepared by mixing 92% by weight of a non-aqueous organic solvent (ethylene carbonate and diethyl carbonate, with a mass ratio of 1:1), 2% by weight of a lithium salt electrolyte (lithium hexafluorophosphate) with a concentration of 1.2 mol / L, 3% by weight of an electrolyte additive, and 3% by weight of an auxiliary additive (vinylene carbonate) to obtain electrolyte 3.

[0034] Comparative Example 1:

[0035] In a glove box under an argon atmosphere with environmental indicators of H2O ≤ 0.1 ppm and O2 ≤ 0.1 ppm, 94% by weight of a non-aqueous organic solvent (ethylene carbonate and diethyl carbonate, with a mass ratio of 1:1) is used, 2% by weight of a lithium salt electrolyte (lithium hexafluorophosphate) with a concentration of 1.2 mol / L is mixed in, and 5% by weight of an auxiliary additive (vinylene carbonate) is added to obtain electrolyte 2.

[0036] The electrolytes prepared in the above Examples 1 - 2 and Comparative Example 1 are added to lithium-ion batteries, and the preparation method of the lithium-ion batteries is as follows:

[0037] Lithium iron manganese phosphate and lithium manganate are selected as the positive electrode material in a ratio of 1:1. The positive electrode material, conductive carbon black, and polyvinylidene fluoride (PVDF) powder are mixed evenly in a ratio of 8:1:1, and then N-methylpyrrolidone (NMP) is added to make a slurry, which is coated on carbon-coated aluminum foil and dried in a vacuum oven, and then punched into a pole piece with a diameter of 14 mm as the positive electrode of a button cell. The loading of the positive electrode of the battery is 10 mg / cm-2. The negative electrode of the battery uses a lithium metal sheet with a thickness of 0.45 mm and a diameter of 16 mm, and polyethylene (PE) is selected as the separator;

[0038] In a glove box under an argon atmosphere with environmental indicators of H2O ≤ 0.1 ppm and O2 ≤ 0.1 ppm, the electrolytes prepared in the above Examples 1 - 2 and Comparative Example 1 are respectively assembled with the positive electrode pole piece, separator, negative electrode, and positive and negative electrode cases, shrapnel, and steel sheets into button cells to obtain Battery 1 (electrolyte 1), Battery 2 (electrolyte 3), and Battery 3 (electrolyte 2).

[0039] Experimental Example 1

[0040] Cyclic test:

[0041] After the above Battery 1, Battery 2, and Battery 3 are left standing for 24 hours, at 25°C, they are cycled 3 times for activation between 3V and 0V at a rate of 0.1C, and then cycled between 3V and 0V at a rate of 0.5C for cyclic test. The test results are as Figure 3 shown.

[0042] FromFigure 3 It can be seen from the data that, compared with the electrolyte 2 of Comparative Example 1, the electrolytes in Examples 1-2 with the addition of electrolyte additives can significantly improve the long-cycle performance of the battery and significantly enhance the cycling of the battery cells, indicating that the electrolytes with the addition of the electrolyte additives of the present invention have good high-temperature cycling performance.

[0043] Experimental Example 2

[0044] Discharge the battery after 600 high-temperature cycles to 2.0V, disassemble the battery cell, and then perform an EDS test on the negative electrode sheet. The test results are shown in Table 1 below:

[0045] Table 1 EDS (energy spectrum) of the negative electrode sheet of three groups of batteries after 600 high-temperature cycles

[0046]

[0047] It can be seen from the data in Table 1 that, compared with the electrolyte of Comparative Example 1, for the electrolytes in Examples 1-2 with the addition of electrolyte additives, the content of Mn metal is only 0.15% and 0.65%, while the content of Mn metal in the electrolyte without the addition of additives is 4.67%. This indicates that the electrolyte with the addition of electrolyte additives can inhibit the dissolution of Mn2+ in the positive electrode and prevent it from being reduced to metallic Mn at the negative electrode (the reaction mechanism of the electrolyte additive is as Figure 2 described).

[0048] Experimental Example 3

[0049] Perform linear sweep voltammetry analysis on Electrolyte 1 and Electrolyte 2. As Figure 1 shown, as the voltage decreases (abscissa), there is no obvious reaction peak in Electrolyte 2 in the range of 0.5V - 1.5V, while for Electrolyte 1 of Example 1, the current increases at 1 - 1.5V, indicating that the electrolyte additive in Electrolyte 1 forms a film preferentially at the negative electrode.

[0050] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. An electrolyte additive suitable for lithium manganese iron phosphate batteries, characterized in that: include: The electrolyte additive is added in an amount of 0.1-3% of the total mass of the electrolyte, and the electrolyte additive contains two benzene rings and two sulfur groups; The two benzene rings of the electrolyte additive maintain a hexagonal planar structure, and six hydrogen atoms are attached to each benzene ring; The two benzene rings of the electrolyte additive are structural units connected by a sulfur atom; The sulfur atom in the electrolyte additive acts as a bridging element and is located between two benzene rings to form a thioether group; The structural formula of the electrolyte additive is:

2. An electrolyte containing the electrolyte additive suitable for lithium manganese iron phosphate battery according to claim 1, characterized in that: The invention comprises the above electrolyte additive, lithium salt electrolyte, non-aqueous organic solvent and auxiliary additive.

3. The electrolyte according to claim 2, characterized in that The added concentration of the lithium salt electrolyte is 1.2 mol / L.

4. The electrolyte according to claim 2, characterized in that The lithium salt electrolyte is one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium difluorobisoxalate, bisfluorosulfonyl imide lithium salt and lithium dioxalate borate.

5. The electrolyte according to claim 2, characterized in that The non-aqueous organic solvent is a mixture of any two of ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate and propylene carbonate.

6. The electrolyte according to claim 2, characterized in that The amount of the non-aqueous organic solvent added is 90-95% of the total mass of the electrolyte.

7. The electrolyte according to claim 2, characterized in that The auxiliary additive is one or more of vinylene carbonate, vinylene carbonate and propylene sulfite.

8. The electrolyte according to claim 2, characterized in that The amount of the auxiliary additive added is 1-5% of the total mass of the electrolyte.

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