An electrolyte for a lithium iron phosphate battery and a lithium iron phosphate battery

Abstract

This application relates to the field of lithium-ion batteries, and discloses an electrolyte for lithium iron phosphate batteries and a lithium iron phosphate battery. The electrolyte includes a solvent, a lithium salt, a first additive, and a second additive. By adding the first additive and the second additive to the electrolyte, the stability of the electrolyte is improved. At the same time, in the lithium iron phosphate system, we surprisingly found that the combined use of the first additive and the second additive has remarkably improved the low-temperature cycle performance of the lithium iron phosphate battery. Meanwhile, its high-temperature, normal-temperature cycle performance and high-temperature storage performance have been significantly improved, and it is significantly superior to the ternary system in terms of the DCIR change rate.

Description

Technical Field

[0001] This application relates to the field of lithium-ion batteries, and particularly to an electrolyte for lithium iron phosphate batteries and a lithium iron phosphate battery. Background Art

[0002] With the decline of traditional energy sources, the development speed of lithium-ion batteries has been accelerating continuously. Countries have placed the development of energy storage batteries and power batteries at the national strategic level, with strong support in terms of matching funds and policies. China is no exception and has even exceeded in this regard. In the past, attention was paid to nickel-metal hydride batteries, but now more attention is focused on lithium iron phosphate batteries. The phosphate-based cathode material of lithium iron phosphate batteries has extremely long cycle life, excellent safety performance, good high-temperature performance, extremely low price, and its low-temperature performance and rate discharge can already reach the level of lithium cobaltate, etc., making it the most promising power battery material.

[0003] Chinese Patent No. 202111199078.7 discloses an electrolyte containing a sulfonic acid phenyl ester compound and a lithium-ion battery. The electrolyte includes a first additive having the structure shown in formula (I) and a second additive having an unsaturated bond.

[0004]

[0005] The first additive of this solution can effectively inhibit and reduce the battery impedance, especially the low-temperature impedance, and further improve the high and low-temperature performance of the battery. Moreover, its overall structure is stable and does not need to be stored at low temperature, and the electrolyte using this compound additive also does not need to be stored at low temperature, and its stability is better than that of the electrolyte containing DTD. The use of a second additive with an unsaturated bond in combination can further enhance the electrochemical performance of the battery under high voltage, especially the cycle performance.

[0006] The existing technology mainly focuses on the application of sulfonic acid phenyl ester compounds in the ternary cathode system. Through repeated research, it is found that the performance improvement of this compound in the ternary cathode system has reached its limit.

[0007] Compared with ternary batteries, lithium iron phosphate batteries have more excellent long cycle performance, high safety performance, low cost, etc. However, they also have the disadvantages of low energy density and poor low-temperature performance, which limit their applications. Therefore, the technical problem to be solved in this application is: how to expand the application scope of the above-mentioned sulfonic acid phenyl ester compound so that it can obtain performance indicators better than those of the ternary cathode system in the lithium iron phosphate cathode system. Summary of the Invention

[0008] The objective of this application is to provide an electrolyte for lithium iron phosphate batteries and a lithium iron phosphate battery. By adding a phenyl sulfonate compound and vinylene carbonate to the electrolyte, the stability of the electrolyte is improved. Meanwhile, in the lithium iron phosphate system, we surprisingly found that the combined use of the phenyl sulfonate compound and vinylene carbonate remarkably improves the low-temperature cycling performance. At the same time, the high-temperature, normal-temperature cycling, and high-temperature storage performances are significantly enhanced, and it is significantly superior to the ternary system in terms of the DCIR change rate.

[0009] To achieve the above objective, this application provides the following technical solutions:

[0010] In the first aspect, an electrolyte for a lithium iron phosphate battery is provided. The electrolyte includes a solvent, a lithium salt, a first additive, and a second additive. The first additive has the general structural formula shown in formula (I);

[0011]

[0012] R1 and R2 are each independently selected from: O, CH2, or a single bond, and at least one of R1 and R2 is selected from O;

[0013] R3, R4, R5, R6, and R7 are each independently selected from: H, a halogen, C 1-8 alkyl, C 2-8 alkenyl, C 3-8 alkynyl, halogen-substituted C 1-8 alkyl, halogen-substituted C 2-8 alkenyl, halogen-substituted C 3-8 alkynyl, or at least one of them;

[0014] The second additive is selected from vinylene carbonate.

[0015] Preferably, R1 and R2 are each independently selected from: O or a single bond;

[0016] R3, R4, R5, R6, and R7 are each independently selected from: H, F, C 1-6 alkyl, C 2-6 alkenyl, C 3-8 alkynyl, F-substituted C 1-6 alkyl, F-substituted C 2-6 alkenyl, F-substituted C 3-6 alkynyl, or at least one of them.

[0017] Preferably, R2 is selected from: O; R3, R4, R5, R6, and R7 are each independently selected from: H, F, methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 2-methyl-1-propyl, 2-butyl, fluoromethyl, fluoroethyl, fluoro-1-propyl, fluoro-2-propyl, fluoro-1-butyl, fluoro-2-methyl-1-propyl, fluoro-2-butyl, vinyl, propenyl, butenyl, fluorovinyl, fluoropropenyl, fluorobutene, propynyl, butynyl, fluoropropynyl, fluorobutynyl, and at least one of the above.

[0018] Preferably, the first additive is selected from any one of the following compounds:

[0019]

[0020]

[0021] Preferably, the addition amount of the first additive accounts for 0.01-10% of the total mass of the electrolyte, and the addition amount of the second additive accounts for 0.1-5% of the total mass of the electrolyte.

[0022] More preferably, the addition amount of the first additive accounts for 0.1-5% of the total mass of the electrolyte, and the addition amount of the second additive accounts for 1-5% of the total mass of the electrolyte.

[0023] It should be understood that the addition amount of the first additive includes, but is not limited to, 0.1%, 0.15%, 0.2%, 0.26%, 0.3%, 0.45%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 2%, 3%, 4%, 5%, and the addition amount of the second additive includes, but is not limited to, 1%, 1.5%, 2%, 2.6%, 3%, 3.4%, 4%, 4.8%, 5%.

[0024] In some embodiments of the present application, the lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium difluoro(oxalato)phosphate, lithium tetrafluoro(oxalato)phosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide. When the lithium salt is selected from the above substances, the mass fraction of the lithium salt in the electrolyte is 5%-20%; preferably 7-18%; more preferably 10-15%.

[0025] In the actual production process, the optional dosage of the above lithium salt includes, but is not limited to: 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, etc.

[0026] In other embodiments of the present application, the lithium salt may also be selected from at least one of lithium difluorophosphate and lithium monofluorophosphate. Considering the low solubility of lithium difluorophosphate and lithium monofluorophosphate in EMC solvent, when the lithium salt is selected from lithium difluorophosphate and / or lithium monofluorophosphate, the mass fraction of the lithium salt in the electrolyte does not exceed 1%, preferably 0.01%-1%, more preferably 0.02%-1%.

[0027] Preferably, the solvent includes a cyclic solvent and / or a linear solvent. The cyclic solvent is selected from at least one of ethylene carbonate, propylene carbonate, γ-butyrolactone, phenyl acetate, 1,4-butanesultone, and 3,3,3-trifluoropropylene carbonate;

[0028] The linear solvent is selected from at least one of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl formate, ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, ethylene glycol dimethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, methyl trifluoroethyl carbonate, (2,2,2)-trifluoroethyl carbonate, 2,2-difluoroethyl acetate, 2,2-difluoroethyl propionate, and 2,2-difluoroethyl methyl carbonate;

[0029] In the electrolyte, by mass percentage, the solvent is 65-94.89%; preferably 70-85%; more preferably 75-85%.

[0030] In the actual production process, the optional amounts of the solvent include but are not limited to: 65%, 70%, 75%, 80%, 85%, 90%, etc.

[0031] Preferably, a third additive is further included. The third additive is selected from at least one of sulfur-containing additives, phosphorus-containing additives, nitrogen-containing additives, and ester additives;

[0032] The sulfur-containing additives are selected from at least one of ethylene sulfate, 1,3-propane sultone, methylene methanedisulfonate, 1,3-propene sultone, methyl propane sultone, N-phenylbis(trifluoromethanesulfonyl)imide, and 3,3,9,9-tetraoxide-2,4,8,10-tetraoxa-3,9-dithiaspiro[5.5]undecane;

[0033] The phosphorus-containing additives are selected from at least one of tris(trimethylsilyl) phosphate, tris(vinyldimethylsilyl) phosphate, and tetramethyl methylenediphosphate;

[0034] The nitrogen-containing additives are selected from at least one of: 2-propyn-1-yl 1H-imidazole-1-carboxylate, hexamethylene diisocyanate, 2-propen-1-yl 1H-imidazole-1-carboxylate, and 2-fluoropyridine;

[0035] The ester additives are selected from at least one of: vinylene ethylene carbonate, fluorinated ethylene carbonate, and trifluoroethoxy ethylene carbonate;

[0036] The dosage of the third additive does not exceed 5% of the total amount of the electrolyte.

[0037] It should be understood that the third additive in this application is an optional additive, and its content in the electrolyte includes but is not limited to: 0%, 0.1%, 0.15%, 0.2%, 0.26%, 0.3%, 0.45%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 2%, 3%, 4%, 5%.

[0038] The electrolyte provided in this application can be prepared by any suitable method known in the art, for example:

[0039] Adding the lithium salt, the first additive, the second additive, and the third additive to the solvent in proportion and mixing can obtain the electrolyte.

[0040] In addition, this application also discloses a lithium iron phosphate battery, which includes:

[0041] A positive electrode sheet;

[0042] A negative electrode sheet;

[0043] A separator; and

[0044] The electrolyte described in the first aspect; the active material of the positive electrode sheet is lithium iron phosphate.

[0045] The beneficial effects of this application are:

[0046] The electrolyte of the present application can effectively improve the low-temperature cycling performance, reduce the battery impedance, and further improve the high-temperature cycling, room-temperature cycling, high-temperature storage, and electrolyte stability performance by using an electrolyte additive containing a compound with the structure shown in formula (I) and vinylene carbonate additive. Among them, the first additive compound has excellent film-forming characteristics. During the first charging process of the battery, it can be reduced at the negative electrode to form a SEI film. The SEI film rich in sulfur elements can greatly improve the ionic conductivity, reduce the battery impedance, and improve the battery cycling performance. After introducing sulfur elements into the relatively sparse SEI film formed by the reduction of the first additive, the introduced second additive further forms a dense SEI film on this basis, avoiding the gas generation problem caused by the deterioration of the imidazole group at high temperatures. Therefore, it can effectively improve the low-temperature cycling, room-temperature cycling, high-temperature cycling, and high-temperature storage performance of the battery. The first additive contains a nitrogen atom with a lone pair of electrons, making the compound show weak Lewis basicity in the electrolyte, capable of forming a hexa-coordinated complex with PF5, reducing the Lewis acidity and reactivity of PF5, thereby effectively inhibiting the increase in the acidity of the electrolyte and the increase in chromaticity caused by the reaction of PF5 with trace impurities in the electrolyte, and further improving the electrolyte stability.

[0047] It should be understood that the electrolyte of the present application is an electrolyte adapted to the lithium iron phosphate positive electrode system. Through experiments, the present application has obtained that compared with the ternary system, the low-temperature cycling performance of the above electrolyte in the lithium iron phosphate system has been astonishingly improved. At the same time, its high-temperature, room-temperature cycling performance, and high-temperature storage performance have been significantly improved. Description of the Drawings

[0048] Figure 1 SEM diagrams of the negative electrodes of the batteries of Comparative Example 5, Comparative Example 2, and Example 1;

[0049] Figure 2 dQ / dV curve diagrams of Comparative Example 5, Comparative Example 2, and Example 1;

[0050] Figure 3 AC impedance diagrams of Comparative Example 5, Comparative Example 2, and Example 1 after 14 days of high-temperature storage; Detailed Embodiments

[0051] The following will clearly and completely describe the present application in combination with the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative efforts shall fall within the protection scope of the present application.

[0052] In the description of this application, it should be noted that for those not specifying specific conditions in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. For reagents or instruments not indicating the manufacturer, they are all conventional products that can be obtained through commercial purchase.

[0053] Preparation of lithium iron phosphate battery:

[0054] 1. Preparation of electrolyte: Ethylene carbonate and ethyl methyl carbonate solvents are mixed at a mass ratio of 1:2. After mixing, LiPF6 is added, and the addition amount of LiPF6 accounts for 13% of the weight of the electrolyte. After the lithium salt is completely dissolved, a first additive and a second additive are added.

[0055] 2. Preparation of positive electrode sheet: Lithium iron phosphate as the positive electrode material, conductive agent SuperP, binder PVDF, and carbon nanotubes (CNT) are mixed evenly at a mass ratio of 95.8:1:2.5:0.7 (NMP as the solvent) to make a lithium-ion battery positive electrode slurry with a certain viscosity, which is coated on carbon-coated aluminum foil used as the current collector, and the coating amount is 35 mg / cm 2 , dried at 85 °C and then cold-pressed; then trimmed, sliced, and slit. After slitting, it is dried at 85 °C under vacuum for 4 hours, and the tab is welded to make a lithium-ion battery positive electrode sheet that meets the requirements.

[0056] 3. Preparation of negative electrode sheet: Graphite, conductive agent SuperP, thickening agent CMC, and binder SBR (styrene-butadiene rubber latex) are made into a slurry at a ratio of 95:1.5:1.5:2.0 by mass, mixed evenly, and the prepared slurry is coated on both sides of the copper foil, and then dried and rolled to obtain a negative electrode sheet, making a lithium-ion battery negative electrode sheet that meets the requirements.

[0057] 4. Preparation of lithium-ion battery: The positive electrode sheet, negative electrode sheet, and separator prepared according to the above process are made into a lithium-ion battery with a thickness of 5.0 mm, a width of 60 mm, and a length of 67 mm by winding process, vacuum baked at 85 °C for 48 hours, and the above electrolyte is injected. After standing for 24 hours, it is charged at a constant current of 0.1C (130 mA) to 3.4V, aged for 24h, then charged at a constant current and constant voltage of 0.2C to 3.65V, discharged at a constant current of 0.2C to 2V, and then the charge and discharge are repeated 1 time at a current of 0.5C and 5 times at a current of 1C respectively. Finally, the battery is charged to 3.65V at a current of 1C to complete the battery production.

[0058] Example 1

[0059] The first additive in this example is the compound 2 with the following structural formula,

[0060]

[0061] The second additive is vinylene carbonate (VC), and the compound 2 accounts for 0.1% of the weight of the electrolyte; the VC accounts for 2.5% of the weight of the electrolyte;

[0062] And prepare a lithium-ion battery according to the preparation method of the above lithium-ion battery.

[0063] Example 2

[0064] It is basically the same as Example 1, except that in this example, the compound 2 accounts for 1% of the weight of the electrolyte; the VC accounts for 2.5% of the weight of the electrolyte.

[0065] Example 3

[0066] It is basically the same as Example 1, except that in this example, the compound 2 accounts for 5% of the weight of the electrolyte; the VC accounts for 2.5% of the weight of the electrolyte.

[0067] Example 4

[0068] It is basically the same as Example 1, except that in this example, the compound 2 accounts for 1% of the weight of the electrolyte; the VC accounts for 0.1% of the weight of the electrolyte.

[0069] Example 5

[0070] It is basically the same as Example 1, except that in this example, the compound 2 accounts for 1% of the weight of the electrolyte; the VC accounts for 1% of the weight of the electrolyte.

[0071] Example 6

[0072] It is basically the same as Example 1, except that in this example, the compound 2 accounts for 1% of the weight of the electrolyte; the VC accounts for 5% of the weight of the electrolyte.

[0073] Example 7

[0074] It is basically the same as Example 2, except that the first additive is compound 6;

[0075]

[0076] Example 8

[0077] It is basically the same as Example 2, except that the first additive is compound 1;

[0078]

[0079] Example 9

[0080] Basically the same as Example 1, except that the lithium salt is selected as LiPF6 and lithium bis(fluorosulfonyl)imide (LiFSI), and the masses of LiPF6 and lithium bis(fluorosulfonyl)imide respectively account for 12% and 1% of the weight of the electrolyte.

[0081] Example 10

[0082] Basically the same as Example 1, except that the solvent is selected as EC:EMC:DEC = 3:5:2.

[0083] Example 11

[0084] Basically the same as Example 1, except that it further includes a third additive, and the third additive is selected as fluoroethylene carbonate (FEC), and its mass accounts for 1% of the weight of the electrolyte.

[0085] Example 12

[0086] Basically the same as Example 1, except that it further includes a third additive, and the third additive is selected as tris(trimethylsilyl) phosphate (TMSP), and its mass accounts for 0.5% of the weight of the electrolyte.

[0087] Comparative Example 1

[0088] Generally the same as Example 1, but the difference is that this comparative example does not contain the first additive and the second additive.

[0089] Comparative Example 2

[0090] Generally the same as Example 1, but the difference is that this comparative example only contains 0.1% of the first additive compound 2 and does not contain the second additive.

[0091] Comparative Example 3

[0092] Generally the same as Example 1, but the difference is that this comparative example only contains 1% of the first additive compound 2 and does not contain the second additive.

[0093] Comparative Example 4

[0094] Generally the same as Example 1, but the difference is that this comparative example does not contain the first additive and only contains 1% of VC.

[0095] Comparative Example 5

[0096] Generally the same as Example 1, but the difference is that this comparative example does not contain the first additive and only contains 2.5% of VC.

[0097] Comparative Example 6

[0098] Generally the same as Example 1, but the difference is that this comparative example does not contain the first additive and only contains 5% of VC.

[0099] Comparative Example 7

[0100] It is substantially the same as Example 1, except that the first additive is 0.1% of the compound shown in Compound 2, and the second additive is 2.5% of vinylene carbonate (VEC).

[0101] Comparative Example 8

[0102] It is substantially the same as Example 1, except that the first additive is 0.1% of Compound I shown below, and the second additive is 2.5% of VC.

[0103]

[0104] Comparative Example 9

[0105] It is substantially the same as Example 1, except that the first additive is 0.5% of lithium difluorophosphate, and the second additive is 2.5% of VC.

[0106] Comparative Example 10

[0107] It is substantially the same as Example 1, except that the first additive is 1% of phenyl methanesulfonate, and the second additive is 2.5% of VC.

[0108] Comparative Example 11

[0109] It is substantially the same as Example 1, except that the first additive is 1% of Compound II shown below, and the second additive is 2.5% of VC.

[0110]

[0111] Comparative Example 12

[0112] It is substantially the same as Example 1, except that the first additive is 1% of Compound III shown below, and the second additive is 2.5% of VC.

[0113]

[0114] Comparative Example 13

[0115] It is substantially the same as Example 1, except that the first additive is 0.1% of Compound 2, and the second additive is 2.5% of 1,3 - propanesultone.

[0116] Comparative Example 14

[0117] It is substantially the same as Example 1, except that the cathode material of the battery is LiNi 0.8 Co 0.1 Mn 0.1 O2, and the electrolyte does not contain any additives.

[0118] Comparative Example 15

[0119] It is substantially the same as Example 1, except that the cathode material of the battery is LiNi 0.8 Co 0.1 Mn 0.1 O2.

[0120] Comparative Example 16

[0121] It is substantially the same as Example 1, except that the cathode material of the battery is LiNi 0.8 Co 0.1 Mn 0.1 O2, and the electrolyte only contains 2.5% of VC and does not contain the first additive.

[0122] Battery performance test

[0123] High-temperature storage: Place the lithium iron phosphate battery after formation and grading in a constant-temperature oven at 60°C and store it for 14 days. Discharge it at a constant current of 1C to 2.0V, and then charge it at a constant current of 1C with constant voltage to 3.65V to test the capacity retention rate and recovery rate. Measure the battery thickness before storage, and calculate the thickness expansion rate by measuring the thickness after 14 days of high-temperature storage.

[0124] Place the ternary battery after formation and grading in a constant-temperature oven at 60°C and store it for 14 days. Discharge it at a constant current of 1C to 3.0V, and then charge it at a constant current of 1C with constant voltage to 4.2V to test the capacity retention rate and recovery rate. Measure the battery thickness before storage, and calculate the thickness expansion rate by measuring the thickness after 14 days of high-temperature storage.

[0125] DCIR performance before and after high-temperature storage: For the lithium iron phosphate battery after formation and grading, before storage and after 14 days of storage at 60°C, charge it at a constant current of 1C with constant voltage to 3.65V at room temperature, let it stand for 5 minutes, then discharge it at a constant current of 1C for 30 minutes, let it stand for 1 hour, and then discharge it at a constant current of 2C for 10 seconds to calculate the DCIR of the battery at 50% SOC.

[0126] For the ternary battery after formation and grading, before storage and after 14 days of storage at 60°C, charge it at a constant current of 1C with constant voltage to 4.2V at room temperature, let it stand for 5 minutes, then discharge it at a constant current of 1C for 30 minutes, let it stand for 1 hour, and then discharge it at a constant current of 2C for 10 seconds to calculate the DCIR of the battery at 50% SOC.

[0127] Low-temperature cycling performance: For the lithium iron phosphate battery after formation and grading, discharge it at a constant current of 0.5C to 2V at -10°C, let it stand for 5 minutes, and then charge it at a constant current of 0.2C with constant voltage to 3.65V for cycling test.

[0128] The ternary battery after formation and grading was discharged at a constant current of 0.5C to 3V at -10°C, left standing for 5 minutes, and then charged at a constant current and constant voltage of 0.2C to 4.2V for cyclic testing.

[0129] Room temperature cyclic performance: The lithium iron phosphate battery after formation and grading was discharged at a constant current of 1C to 2V at 25°C, left standing for 5 minutes, and then charged at a constant current and constant voltage of 1C to 3.65V for cyclic testing.

[0130] The ternary battery after formation and grading was discharged at a constant current of 1C to 3V at 25°C, left standing for 5 minutes, and then charged at a constant current and constant voltage of 1C to 4.2V for cyclic testing.

[0131] High temperature cyclic performance: The lithium iron phosphate battery after formation and grading was discharged at a constant current of 1C to 2V at 55°C, left standing for 5 minutes, and then charged at a constant current and constant voltage of 1C to 3.65V for cyclic testing.

[0132] The ternary battery after formation and grading was discharged at a constant current of 1C to 3V at 45°C, left standing for 5 minutes, and then charged at a constant current and constant voltage of 1C to 4.2V for cyclic testing.

[0133] The test results can be referred to Table 1 below:

[0134]

[0135] Reference Figure 1 , Figure 1 shows the SEM images of the battery negative electrodes of Comparative Example 5, Comparative Example 2 and Example 1.

[0136] From Figure 1 it can be seen that the SEI film formed by a single vinylene carbonate without sulfur element is relatively smooth, the SEI film rich in sulfur element formed by a single first additive is relatively rough, while the SEI film formed after adding the two additives is not only rich in sulfur element but also the whole SEI film is smooth.

[0137] Reference Figure 2 , Figure 2 is the dQ / dV curve graph of Comparative Example 5, Comparative Example 2 and Example 1;

[0138] From Figure 2 it can be seen that the first additive is preferentially reduced at about 2.5V, earlier than 2.7V of VC, and at the same time a second reduction peak appears at about 2.8V. The second reduction peak and its products may be the main sources of gas generation. After adding the two additives in Example 1, the reduction peak at about 2.5V still exists, while the reduction peak at about 2.8V is inhibited.

[0139] Reference Figure 3 , Figure 3AC impedance diagrams of Comparative Example 5, Comparative Example 2 and Example 1 after 14 days of high-temperature storage;

[0140] From Figure 3 It can be seen that when the first additive compound is used alone in the lithium iron phosphate battery system, it can significantly reduce the battery R CT / R SEI impedance (charge transfer impedance or SEI film impedance, corresponding to the semicircle). However, in the absence of VC addition, gas will be generated at high temperature, causing the battery R b to increase (internal battery impedance, corresponding to the abscissa intercept). When the first additive is used in combination with VC, not only does the battery R b not increase, but also significantly reduces the battery R CT / R SEI impedance, showing an excellent impedance reduction effect.

[0141] Referring to Table 1, at least the following conclusions can be obtained:

[0142] 1. Referring to Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 5, it can be found that;

[0143] In terms of the capacity retention rate after 1500 cycles at 25°C and the capacity retention rate after 1500 cycles at 55°C (45°C), VC plays a major role;

[0144] In terms of the DCIR change rate, the DCIR of Example 1 increased by 3.8 mΩ, that of Comparative Example 1 increased by 8.6 mΩ, and that of Comparative Example 5 increased by 7.9 mΩ. It can be seen that the first additive of the present application plays a leading role in cooperation with VC in improving DCIR;

[0145] It should be particularly noted that in terms of the number of cycles at -10°C with a capacity retention rate of 80%, when using VC alone, the performance will deteriorate significantly, indicating that the first additive plays a leading role in improving low-temperature cycling.

[0146] 2. Referring to Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 5, Comparative Example 14, Comparative Example 15, and Comparative Example 16, it can be found that:

[0147] In the ternary system, the improvements in terms of the capacity retention rate after 1500 cycles at 25°C, the capacity retention rate after 1500 cycles at 55°C (45°C), and the DCIR change rate are not particularly obvious, playing a relatively balanced improvement role. The addition of 2.5% VC has a slight negative impact on the ternary battery, and the amount of VC added to the ternary battery system should not be too much.

[0148] In terms of the number of cycles at -10°C with a capacity retention rate of 80%, the ternary system also does not show obvious advantages.

[0149] It can be concluded that the combination of the first additive and VC has significant advantages in improving low-temperature cycling and the DCIR change rate for the lithium iron phosphate system.

[0150] Referring to Comparative Example 2, Comparative Example 3, Example 1, and Example 2, it can be found that in the absence of VC, an increase in the dosage of the first additive will lead to a deterioration in the capacity retention rate after 1500 cycles at 25°C and the capacity retention rate after 1500 cycles at 55°C (45°C), indicating that the first additive has a negative effect on the capacity retention rate after 1500 cycles at 25°C and the capacity retention rate after 1500 cycles at 55°C (45°C).

[0151] Similarly, referring to Comparative Examples 4-6, it can be found that in the absence of the first additive, VC also has a negative effect on the number of cycles at -10°C with an 80% capacity retention rate.

[0152] By comparing Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 7, and Comparative Example 13, it can be found that VEC and 1,3-propane sultone have a more negative effect on the number of cycles at -10°C with an 80% capacity retention rate compared to VC. This shows that although using VC alone has an obvious negative effect in this performance aspect, after compounding with the first additive, this negative effect can be basically eliminated. Therefore, the first additive and VC are a very excellent combination in terms of the number of cycles at -10°C with an 80% capacity retention rate.

[0153] By comparing Example 1 and Comparative Example 8, it can be found that although Compound 7 has a very similar structure to Compound 2 of the present application, it has no effect on the number of cycles at -10°C with an 80% capacity retention rate.

[0154] By comparing Example 2 with Comparative Examples 11 and 12, it can be found that although Compounds 8 and 9 have many common technical features with Compound 2 of the present application, they have no effect on the number of cycles at -10°C with an 80% capacity retention rate.

[0155] Through the above analysis, we can know that not all compounds contained in 202111199078.7 are applicable to the lithium iron phosphate system. In other words, only the compound of the present application compounded with VC is applicable to the lithium iron phosphate system.

[0156] By comparing Example 2 with Comparative Examples 9 and 10, it can be found that the compounding of conventional low-temperature additives or film-forming additives with VC cannot improve the performance of the present application in terms of the number of cycles at -10°C with an 80% capacity retention rate.

[0157] Summary:

[0158] It can be proven by the above-mentioned examples and comparative examples that:

[0159] Conclusion 1: VC is the main factor improving the capacity retention rate after 1500 cycles at 25°C and the capacity retention rate after 1500 cycles at 55°C (45°C); it is the negative factor of the first additive;

[0160] Conclusion 2: The first additive is the main factor improving the number of cycles with 80% capacity retention rate at -10°C; VC is the negative factor;

[0161] Conclusion 3: The compounding of the first additive and VC synergistically improves the DCIR change rate.

[0162] Conclusion 4: In terms of the capacity retention rate after 1500 cycles at 25°C and the capacity retention rate after 1500 cycles at 55°C (45°C), VC can eliminate the negative impact of the first additive, and other similar additives cannot eliminate the negative impact of the first additive;

[0163] In terms of the number of cycles with 80% capacity retention rate at -10°C, the first additive can eliminate the negative impact of VC, and other similar additives cannot eliminate the negative impact of VC.

[0164] It can be confirmed from the above conclusions that: in terms of improving the capacity retention rate after 1500 cycles at 25°C, the capacity retention rate after 1500 cycles at 55°C (45°C), the number of cycles with 80% capacity retention rate at -10°C, and the DCIR change rate, the first additive and VC are the only optimal compounding choices.

Claims

1. An electrolyte for a lithium iron phosphate battery, characterized in that, The electrolyte includes a solvent, a lithium salt, a first additive, and a second additive, and the first additive has a general structural formula shown in formula (I); In formula (I), R1 and R2 are each independently selected from: O, CH2 or a single bond, and at least one of R1 and R2 is selected from O; R3, R4, R5, R6, and R7 are each independently selected from: H, halogen, C 1-8 alkyl, C 2-8 alkenyl, C 3-8 alkynyl, halogen-substituted C 1-8 alkyl, halogen-substituted C 2-8 alkenyl, halogen-substituted C 3-8 alkynyl, and at least one of them; The second additive is selected from vinylene carbonate.

2. The electrolyte for lithium iron phosphate batteries according to claim 1, characterized in that, R1 and R2 are each independently selected from: O or a single bond; R3, R4, R5, R6, and R7 are each independently selected from: H, F, C 1-6 alkyl, C 2-6 alkenyl, C 3-8 alkynyl, F-substituted C 1-6 alkyl, F-substituted C 2-6 alkenyl, F-substituted C 3-6 at least one of alkynyl.

3. The electrolyte for a lithium iron phosphate battery according to claim 2, wherein R2 is selected from: O; R3, R4, R5, R6, and R7 are each independently selected from: H, F, methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 2-methyl-1-propyl, 2-butyl, fluoromethyl, fluoroethyl, fluorinated 1-propyl, fluorinated 2-propyl, fluorinated 1-butyl, fluorinated 2-methyl-1-propyl, fluorinated 2-butyl, vinyl, propenyl, butenyl, fluorinated vinyl, fluorinated propenyl, fluorinated butenyl, propynyl, butynyl, fluorinated propynyl, fluorinated butynyl, etc. at least one.

4. The electrolyte for a lithium iron phosphate battery according to claim 1, wherein The first additive is selected from any one of the following compounds:

5. The electrolyte for lithium iron phosphate batteries according to claim 1, characterized in that, The addition amount of the first additive accounts for 0.01 - 10% of the total mass of the electrolyte; the addition amount of the second additive accounts for 0.1 - 5% of the total mass of the electrolyte.

6. The electrolyte for a lithium iron phosphate battery according to claim 5, characterized in that, The addition amount of the first additive accounts for 0.1 - 5% of the total mass of the electrolyte; the addition amount of the second additive accounts for 1 - 5% of the total mass of the electrolyte.

7. The electrolyte for a lithium iron phosphate battery according to claim 1, wherein, The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium difluoro(oxalato)phosphate, lithium tetrafluoro(oxalato)phosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, etc., and the mass fraction of the lithium salt in the electrolyte is 5% - 20%.

8. The electrolyte for lithium iron phosphate batteries according to claim 7, characterized in that, The mass fraction of the lithium salt in the electrolyte is 7% - 18%.

9. The electrolyte for lithium iron phosphate batteries according to claim 8, wherein, The mass fraction of the lithium salt in the electrolyte is 10% - 15%.

10. The electrolyte for lithium iron phosphate battery according to claim 1, characterized in that, The lithium salt is selected from at least one of lithium difluorophosphate and lithium monofluorophosphate, and the mass fraction of the lithium salt in the electrolyte is 0.01% - 1%.

11. The electrolyte for a lithium iron phosphate battery according to claim 1, characterized in that, The solvent includes at least one of a cyclic solvent and a linear solvent; The cyclic solvent is selected from: at least one of ethylene carbonate, propylene carbonate, γ-butyrolactone, phenyl acetate, 1,4-butanesultone, and 3,3,3-trifluoropropylene carbonate; The linear solvent is selected from: at least one of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl formate, ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, ethylene glycol dimethyl ether, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, methyl trifluoroethyl carbonate, (2,2,2)-trifluoroethyl carbonate, 2,2-difluoroethyl acetate, 2,2-difluoroethyl propionate, and 2,2-difluoroethyl methyl carbonate; In the electrolyte, by mass percentage, the content of the solvent is 65 - 94.89%.

12. The electrolyte for a lithium iron phosphate battery according to claim 1, characterized in that, It further includes a third additive, and the third additive is selected from: at least one of sulfur-containing additives, phosphorus-containing additives, nitrogen-containing additives, and ester additives; The sulfur-containing additives are selected from at least one of: ethylene sulfate, 1,3-propane sultone, methylene methanedisulfonate, 1,3-propene sultone, methyl propane sultone, N-phenylbis(trifluoromethanesulfonyl)imide, 3,3,9,9-tetraoxide-2,4,8,10-tetraoxa-3,9-dithiaspiro[5.5]undecane; The phosphorus-containing additives are selected from at least one of: tris(trimethylsilyl) phosphate, tris(vinyldimethylsilyl) phosphate, tetramethyl methylenediphosphate; The nitrogen-containing additives are selected from at least one of: 2-propyn-1-yl 1H-imidazole-1-carboxylate, hexamethylene diisocyanate, 2-propen-1-yl 1H-imidazole-1-carboxylate, 2-fluoropyridine; The ester additives are selected from at least one of: vinylene ethylene carbonate, fluorinated ethylene carbonate, trifluoroethoxy ethylene carbonate; The dosage of the third additive does not exceed 5% of the total amount of the electrolyte.

13. A lithium iron phosphate battery, characterized in that, The lithium iron phosphate battery includes: a positive electrode sheet; a negative electrode sheet; a separator; and the electrolyte for a lithium iron phosphate battery according to any one of claims 1 to 12; The active material of the positive electrode sheet is lithium iron phosphate.

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