Electrolyte for lithium manganese iron phosphate battery and lithium manganese iron phosphate battery

Abstract

The electrolyte comprises a non-aqueous organic solvent, a lithium salt and an additive, the additive comprises an unsaturated cyclic sulfimide salt and a sulfur-containing silane compound, the structure of the unsaturated cyclic sulfimide salt is as shown in a formula 1, the structure of the sulfur-containing silane compound is as shown in a formula 2, and the structure of the sulfur-containing silane compound is as shown in a formula 3. The structure of the sulfur-containing silane compound is shown as a formula 2 or a formula 3, M is alkali metal, R5 is selected from C1-C6 alkyl or * represents a connecting end; r < 1 > to R < 4 > and R < 6 > to R < 7 > are respectively and independently selected from C1-C6 alkyl groups; n is an integer of 2-6, and m is an integer of 1-4. The electrolyte can improve the ionic conductivity, effectively form a stable SEI film and a CEI film, and effectively inhibit manganese dissolution and capture manganese ions, so that the lithium manganese iron phosphate battery has relatively good high-temperature storage performance and high-temperature cycle performance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion batteries, and particularly relates to an electrolyte for a lithium manganese iron phosphate battery and a lithium manganese iron phosphate battery. BACKGROUND

[0002] In recent years, with the rapid development of new energy vehicles, lithium iron phosphate (LiFePO4, LFP) positive materials have become one of the mainstream choices in the field of power batteries due to their high safety, long cycle life and low cost. However, due to the low discharge voltage platform (about 3.4 V vs. Li + / Li) and the specific capacity close to the theoretical value, the energy density of lithium iron phosphate batteries has reached the limit and cannot meet the market demand for higher endurance mileage. Therefore, developing new positive materials with high energy density and excellent stability has become a research hotspot in the industry.

[0003] Based on lithium iron phosphate, lithium manganese iron phosphate (LiFe 1- x Mn x PO4, 0<x<1, LFMP) composite material is considered as the next generation of LFP upgrade route. The introduction of Mn introduces a high voltage platform of about 4.1V (vs. Li + / Li) in the material, which significantly improves the energy density of the material, while retaining the stability and safety advantages of the olivine structure. Therefore, lithium manganese iron phosphate has become a high-voltage positive material with practical potential and has become the promotion route of major battery and vehicle manufacturers.

[0004] LiFe 1-x Mn x PO4, x represents the doping ratio of manganese, which is crucial to the properties of lithium manganese iron phosphate. On the one hand, the increase of manganese in lithium manganese iron phosphate material can increase the contribution of 4.1V high voltage platform, which can improve the overall energy density of the material; on the other hand, due to the existence of Jahn-Teller distortion, the transformation between Mn 3+ / Mn 3+ / Mn 4+ during charging and discharging causes the structure of manganese-based positive material to be unstable, resulting in particle breakage of the positive material during the cycle process, serious manganese dissolution behavior, and inability to achieve stable cycling.

[0005] Therefore, there is an urgent need for an electrolyte for a lithium manganese iron phosphate battery and a lithium manganese iron phosphate battery to solve the problems of the prior art. SUMMARY

[0006] In view of the above problems, the purpose of the present application is to provide an electrolyte for a lithium iron manganese phosphate battery and a lithium iron manganese phosphate battery, which can improve ion conductivity, effectively form stable SEI and CEI films, effectively inhibit manganese dissolution and capture manganese ions, so that the lithium iron manganese phosphate battery has good high-temperature storage performance and high-temperature cycle performance.

[0007] To achieve the above purpose, the first aspect of the present application provides an electrolyte for a lithium iron manganese phosphate battery, comprising a non-aqueous organic solvent, a lithium salt and an additive, the additive comprising an unsaturated cyclic sulfimide salt and a sulfur-containing silane compound, the structure of the unsaturated cyclic sulfimide salt is shown in formula 1, and the structure of the sulfur-containing silane compound is shown in formula 2 or formula 3,

[0008] wherein M is an alkali metal, R5 is selected from C1-C6 alkyl or , * represents a connecting end; R1-R 4、 R6-R7 are each independently selected from C1-C6 alkyl; n is an integer of 2-6, and m is an integer of 1-4.

[0009] Compared with the prior art, the additive in the electrolyte of the present application comprises an unsaturated cyclic sulfimide salt, which has a higher HOMO energy level and can be oxidized at the positive electrode to form a stable CEI protective film in preference to solvent molecules; at the same time, the low LUMO energy level enables it to be reduced at the negative electrode to form a tough, uniform and stable SEI protective film in preference to solvent molecules; at the same time, the SEI protective film is rich in sulfate / sulfite components, which can effectively increase the mechanical strength of the SEI film and reduce the interface gas production phenomenon, thereby significantly improving the high-temperature storage performance of the lithium iron manganese phosphate battery. However, the viscosity of the unsaturated cyclic sulfimide salt is large, which reduces the migration rate of lithium ions in the electrolyte and affects the electrochemical performance (such as cycle stability) of the battery. Based on this, the additive of the present application further comprises a sulfur-containing silane compound, which has a large solubility in the organic solvent, which helps to reduce the viscosity of the organic solvent and improve the migration rate of lithium ions in the electrolyte; at the same time, the sulfur atom in the sulfur-containing silane compound provides a lone pair of electrons to coordinate with manganese ions to inhibit manganese dissolution; and the alkoxy group of silane can be hydrolyzed to silanol, which further captures manganese ions through hydrogen bonds or covalent bonds to reduce the capacity decay caused by positive electrode dissolution and increase the stability of the positive electrode material, thereby improving the high-temperature cycle performance of the lithium iron manganese phosphate battery.

[0010] Further, n is 2, 3, 4, 5 or 6, and m is 1, 2, 3 or 4.

[0011] Further, the unsaturated cyclic sulfimide salt of the present application is selected from at least one of compounds 1-5: .

[0012] Specifically, the acetylsulfamic acid compound is dissolved in deionized water, the reactant 1 (lithium carbonate) is added in batches, and after the reaction is completed with sufficient stirring, white precipitate is produced, and after filtration and recrystallization, compound 1 is obtained. The synthesis route of compound 1 is as follows:

[0013] Correspondingly, the synthesis routes of compounds 2-5 can be carried out by referring to the synthesis route of compound 1, only the type of reactant 1 is different; for example, adjusting the reactant 1 to sodium carbonate to obtain compound 2; adjusting the reactant 1 to potassium carbonate to obtain compound 3; adjusting the reactant 1 to cesium carbonate to obtain compound 4; and adjusting the reactant 1 to rubidium carbonate to obtain compound 5.

[0014] Further, the sulfur-containing silane compound of the present application is selected from at least one of compounds 6-10: .

[0015] Among them, the cas number of compound 6 is 862472-15-9; the cas number of compound 7 is 862472-15-9; the cas number of compound 8 is 1934391-16-8; the cas number of compound 9 is 2756582-02-0; and the cas number of compound 10 is 862472-16-0.

[0016] Further, the mass percentage of the unsaturated cyclic sulfonimide salt in the electrolyte is 0.05-5%; preferably, the mass percentage of the unsaturated cyclic sulfonimide salt in the electrolyte is 0.1-2%; more preferably, the mass percentage of the unsaturated cyclic sulfonimide salt in the electrolyte is 0.1-1%. As an example, the mass percentage of the unsaturated cyclic sulfonimide salt in the electrolyte is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, but not limited to the listed values, other values not listed in this range are also applicable.

[0017] Further, the mass percentage of the sulfur-containing silane compound in the electrolyte solution is 0.05-5%. Preferably, the mass percentage of the sulfur-containing silane compound in the electrolyte solution is 0.1-4%; more preferably, the mass percentage of the sulfur-containing silane compound in the electrolyte solution is 0.5-2%. As an example, the mass percentage of the sulfur-containing silane compound in the electrolyte solution is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, but is not limited to the listed values, and other values not listed in the range are also applicable.

[0018] Further, the lithium salt of the present application is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium trifluoromethylsulfonate (LiCF3SO3), lithium bis-trifluoromethylsulfonimide (LiTFSI), lithium bis(oxalato)borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium fluorosulfonate (LiSO2F), lithium bis(oxalato)borate (C4BLiO8), lithium difluoro(oxalato)borate, lithium lower aliphatic carboxylate, lithium difluorodioxalato phosphate, and lithium bisfluorosulfonimide. Further, the lithium lower aliphatic carboxylate includes, but is not limited to, lithium chloroborane, lithium tetraphenylborate, lithium imide salt, etc. As an example, the lithium salt is lithium hexafluorophosphate (LiPF6), but is not limited thereto. Further, the lithium salt is a mixture of lithium hexafluorophosphate and lithium bis(oxalato)borate, but is not limited thereto.

[0019] Further, the mass percentage of the lithium salt in the electrolyte solution is 5-25%; further, the mass percentage of the lithium salt in the electrolyte solution is 8-20%, more preferably, the mass percentage of the lithium salt in the electrolyte solution is 10-15%; as an example, the mass percentage of the lithium salt in the electrolyte solution can be, but is not limited to, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%.

[0020] Further, the non-aqueous organic solvent of the present application is selected from at least one of gamma-butyrolactone, gamma-valerolactone, delta-valerolactone, methyl acetate (MA), ethyl acetate (EA), ethyl propionate (EP), butyl acetate (n-Ba), propyl propionate (n-PP), butyl propionate, ethylene carbonate (EC), propylene carbonate, butylene carbonate (BC), pentylene carbonate, vinylene carbonate (VC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), methyl n-propyl carbonate, ethyl n-propyl carbonate, propylene carbonate (PC), 1,3-dioxolane (DOL), 1,4-dioxolane (DX), crown ether, tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-CH3-THF), 2-trifluoromethyltetrahydrofuran (2-CF3-THF), dimethoxymethane, diethoxymethane, ethoxymethoxymethane, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, diethylene glycol dimethyl ether.

[0021] Further, the mass percentage of the non-aqueous organic solvent in the electrolyte of the present application is 65-90%. Preferably, the mass percentage of the non-aqueous organic solvent in the electrolyte is 75-89%; more preferably, the mass percentage of the non-aqueous organic solvent in the electrolyte is 78-88%. Specifically, the mass percentage of the non-aqueous organic solvent in the electrolyte can be, but is not limited to, 67%, 70%, 78%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%.

[0022] Further, the electrolyte of the present application further comprises an additive selected from at least one of fluoroethylene carbonate (FEC), vinylene carbonate (VC), 1,3-propane sultone (PS), ethylene sulfate (DTD), 1,3-propanediol cyclic sulfate (PCS), 1,4-butane sultone, triallyl phosphate (TAP), succinic anhydride. Among them, VC can be preferentially reduced at the negative electrode to form a flexible SEI film rich in polycarbonate, which is complementary to the rigid component of LiF / Li2SO3 generated by the decomposition of unsaturated sulfonimide salt, to construct a composite interface structure of rigidity and flexibility, thereby significantly improving the overall electrochemical performance of the battery.

[0023] Further, the mass percentage of the additive in the electrolyte of the present application is 0.5-6%. Preferably, the mass percentage of the additive in the electrolyte is 1-5%; more preferably, the mass percentage of the additive in the electrolyte is 2-4%. Specifically, the mass percentage of the additive in the electrolyte can be, but is not limited to, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%.

[0024] Accordingly, the second aspect of the present application provides a lithium iron manganese phosphate battery, comprising a positive electrode, a negative electrode and the above-mentioned electrolyte for lithium iron manganese phosphate battery, the active material of the positive electrode is LiFe 1-x Mn x PO4, 0 < x < 1. The unsaturated cyclic sulfonimide salt in the electrolyte can effectively form stable SEI film and CEI film; at the same time, the sulfur-containing silane compound can improve the migration rate of lithium ions in the electrolyte, inhibit manganese dissolution and capture manganese ions, so that the lithium iron manganese phosphate battery has good high-temperature storage performance and high-temperature cycle performance.

[0025] Further, x represents the doping ratio of manganese, specifically, the manganese-iron ratio can be but is not limited to 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1.

[0026] Further, x≥0.5 and the maximum charging voltage is 4.3V. The electrolyte can effectively inhibit the high manganese content (x≥0.5) LiFe 1-x Mn x PO4 positive electrode material in the cycle process, such as Mn 3+ caused by Jahn-Teller distortion, particle rupture and manganese dissolution, etc., thereby ensuring that the lithium iron manganese phosphate battery still has good storage performance and cycle stability in a high-temperature environment.

[0027] Further, the active material of the negative electrode of the present application is selected from at least one of artificial graphite, natural graphite, lithium titanate, silicon-carbon composite material and silicon monoxide. DETAILED DESCRIPTION

[0028] In order to better illustrate the purpose, technical scheme and beneficial effects of the present application, the present application will be further described below in combination with specific examples. It should be noted that the following implementation of the method is a further explanation and description of the present application, and should not be regarded as a limitation of the present application.

[0029] Example 1 (1) Preparation of electrolyte In an argon atmosphere, in a vacuum glove box with water content <1ppm, ethylene carbonate (EC), methyl ethyl carbonate (EMC) and ethyl propionate (EP) are mixed according to the weight ratio of EC: EMC: EP = 7:2:3 to prepare 85.8g of non-aqueous organic solvent, then 0.2g of compound 1 and 0.5g of compound 6 are added, dissolved and stirred thoroughly, then 13.5g of LiPF6 is added, mixed uniformly to obtain the electrolyte.

[0030] (2) Preparation of positive electrode sheet The lithium iron manganese phosphate material LiMn 0.5Fe 0.5 PO4, binder PVDF and conductive agent Super P were mixed in a mass ratio of 95:1:4 to prepare a positive electrode slurry with a certain viscosity. After the mixed slurry was coated on both sides of the aluminum foil, it was dried and rolled to obtain a positive electrode sheet.

[0031] (3) Preparation of negative electrode sheet The negative electrode graphite material, binder PVDF and conductive agent Super P were mixed in a mass ratio of 90:2:8 to prepare a negative electrode slurry with a certain viscosity. After the mixed slurry was coated on both sides of the copper foil, it was dried and rolled to obtain a negative electrode sheet.

[0032] (4) Preparation of lithium manganese iron phosphate battery The positive electrode sheet, separator and negative electrode sheet were stacked in order, and then stacked as needed. After the tab was welded, it was placed in an aluminum plastic film battery outer package. The above prepared electrolyte was injected into the dried bare cell, and then vacuum packaging, standing, formation (0.05C constant current charging to 3.0V, then 0.1C constant current charging to 4.3V), capacity test and other processes were carried out in sequence. Finally, a 1Ah soft package lithium manganese iron phosphate battery was obtained.

[0033] The formulations of the non-aqueous electrolyte of Examples 1-17 and Comparative Examples 1-3 are shown in Table 1. The preparation of the electrolyte and the steps of preparing the lithium manganese iron phosphate battery of Examples 2-17 and Comparative Examples 1-3 are the same as those of Example 1.

[0034] Table 1 Formulation of electrolyte of each example and comparative example

[0035] The lithium manganese iron phosphate batteries prepared in Examples 1-17 and Comparative Examples 1-3 were subjected to high temperature storage and high temperature cycle test, respectively. The specific test conditions are as follows, and the performance test results are shown in Table 2.

[0036] High temperature storage performance test Under normal temperature (25℃) conditions, the lithium manganese iron phosphate battery was subjected to one 0.3C / 0.3C charging and discharging (the battery discharge capacity was recorded as C0), and the upper limit voltage was 4.3V. The battery was placed in a 60℃ oven for 30d, and then taken out and placed in a 25℃ environment for 0.3C discharging. The discharge capacity was recorded as C1. Then the lithium manganese iron phosphate battery was subjected to one 0.3C / 0.3C charging and discharging (the battery discharge capacity was recorded as C2). The capacity retention rate and the capacity recovery rate of the lithium manganese iron phosphate battery were calculated using the following formula: Capacity retention rate = (C1 / C0) x 100%; Capacity recovery rate = (C2 / C0) x 100%; High temperature cycling performance test The lithium manganese iron phosphate battery was placed in a 45°C constant temperature oven for 30 minutes, and the lithium manganese iron phosphate battery was allowed to reach a constant temperature. The battery was charged at 1C constant current to a voltage of 4.3V, then charged at 4.3V constant voltage to a current of 0.05C, and then discharged at 1C constant current to a voltage of 2.5V. The first cycle discharge capacity of the battery was recorded as C0, which was one charge-discharge cycle. Then, 1C / 1C charging and discharging were carried out at 45°C for 500 cycles, and the discharge capacity was recorded as C1. The capacity retention rate of the lithium manganese iron phosphate battery was calculated using the following formula.

[0037] Capacity retention rate = (C1 / C0) x 100%.

[0038] Table 2 Test results of lithium manganese iron phosphate batteries

[0039] As can be seen from Table 2, compared with Comparative Examples 1-3, the lithium ion batteries of Examples 1-9 have better high-temperature storage and high-temperature cycle performance. This is because the additive in the electrolyte of the application includes an unsaturated cyclic sulfimide salt, which has a high HOMO energy level and can be oxidized at the positive electrode to form a stable CEI protective film prior to solvent molecules; at the same time, the low LUMO energy level enables it to be reduced at the negative electrode to form a tough, uniform and stable SEI protective film prior to solvent molecules; at the same time, the SEI protective film is rich in sulfate / sulfite components, which can effectively increase the mechanical strength of the SEI film and reduce the interface gas production phenomenon, thereby significantly improving the high-temperature storage performance of the lithium manganese iron phosphate battery. However, the viscosity of the unsaturated cyclic sulfimide salt is large, which reduces the migration rate of lithium ions in the electrolyte and affects the electrochemical performance (such as cycle stability) of the battery. Based on this, the additive of the application also includes a sulfur-containing silane compound, which has a large solubility in organic solvents, which helps to reduce the viscosity of the organic solvent and increase the migration rate of lithium ions in the electrolyte; at the same time, the sulfur atom in the sulfur-containing silane compound provides a lone pair of electrons to coordinate with manganese ions to inhibit manganese dissolution; and the alkoxy group of the silane can be hydrolyzed to silanol, which further captures manganese ions through hydrogen bonds or covalent bonds to reduce the capacity decay caused by positive electrode dissolution and increase the stability of the positive electrode material, thereby improving the high-temperature cycle performance of the lithium manganese iron phosphate battery.

[0040] As can be seen from the comparison of Examples 1 and 15-17, the introduction of the additive can further improve the high-temperature cycle performance and high-temperature storage performance of the lithium manganese iron phosphate battery.

[0041] Comparing example 15 to examples 16 and 17, it can be seen that the lithium iron manganese phosphate battery of example 15 has relatively better high-temperature cycle performance and high-temperature storage performance, because VC can be preferentially reduced at the negative electrode to form a flexible SEI film rich in polycarbonate, which is complementary to the rigid components of LiF / Li2SO3 generated by the decomposition of unsaturated sulfonimide salt, thereby constructing a composite interface structure of rigidity and flexibility, and significantly improving the overall electrochemical performance of the battery.

[0042] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application, and are not intended to limit the scope of protection of the present application. Although the present application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the present application.

Claims

1. An electrolyte for lithium manganese iron phosphate batteries, comprising a non-aqueous organic solvent, a lithium salt, and additives, characterized in that, The additives include unsaturated cyclic sulfonyl imide salts and sulfur-containing silane compounds. The structure of the unsaturated cyclic sulfonyl imide salt is shown in Formula 1, and the structure of the sulfur-containing silane compound is shown in Formula 2 or Formula 3. Wherein, M is an alkali metal, and R5 is selected from C1~C6 alkyl groups or... * indicates a connection end; R1~R 4、 R6 to R7 are each independently selected from C1 to C6 alkyl groups; n is an integer from 2 to 6, and m is an integer from 1 to 4.

2. The electrolyte for lithium manganese iron phosphate batteries as described in claim 1, characterized in that, The unsaturated cyclic sulfonamide salt is selected from at least one of compounds 1 to 5: 。 3. The electrolyte for lithium manganese iron phosphate batteries as described in claim 1, characterized in that, The sulfur-containing silane compound is selected from at least one of compounds 6 to 10: 。 4. The electrolyte for lithium manganese iron phosphate batteries as described in claim 1, characterized in that, The unsaturated cyclic sulfonamide salt has a mass percentage of 0.05-5% in the electrolyte; the sulfur-containing silane compound has a mass percentage of 0.05-5% in the electrolyte.

5. The electrolyte for lithium manganese iron phosphate batteries as described in claim 1, characterized in that, The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(oxalate-borate), lithium difluorophosphate, lithium fluorosulfonate, lithium bis(oxalate-borate), lithium difluorooxalate-borate, lithium lower aliphatic carboxylic acids, lithium difluorobis(oxalate-phosphate), and lithium bis(oxalate-imide).

6. The electrolyte for lithium manganese iron phosphate batteries as described in claim 1, characterized in that, The non-aqueous organic solvent is selected from at least one of γ-butyrolactone, γ-valerolactone, δ-valerolactone, methyl acetate, ethyl acetate, ethyl propionate, butyl acetate, propyl propionate, butyl propionate, ethylene carbonate, propylene carbonate, butyl carbonate, pentylenetene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl n-propyl carbonate, ethyl n-propyl carbonate, propylene carbonate, 1,3-dioxane, 1,4-dioxane, crown ether, tetrahydrofuran, 2-methyltetrahydrofuran, 2-trifluoromethyltetrahydrofuran, dimethoxymethane, diethoxymethane, ethoxymethoxymethane, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, and diethylene glycol dimethyl ether.

7. The electrolyte for lithium manganese iron phosphate batteries as described in claim 1, characterized in that, It also includes additives selected from at least one of fluoroethylene carbonate, vinylene carbonate, 1,3-propanesulfonate lactone, vinyl sulfate, 1,3-propanediol cyclosulfonate, 1,4-butanesulfonate lactone, triallyl phosphate, and succinic anhydride.

8. A lithium manganese iron phosphate battery, characterized in that, The battery includes a positive electrode, a negative electrode, and an electrolyte for a lithium manganese iron phosphate battery as described in any one of claims 1 to 7, wherein the active material of the positive electrode is LiFe. 1-x Mn x PO4, 0 <x<1。 9. The lithium manganese iron phosphate battery as described in claim 8, characterized in that, x≥0.5 and the maximum charging voltage is 4.3V.

10. The lithium manganese iron phosphate battery as described in claim 8, characterized in that, The active material of the negative electrode is selected from at least one of artificial graphite, natural graphite, lithium titanate, silicon-carbon composite material, and silicon suboxide.

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