Electrolyte for lithium iron manganese phosphate battery, preparation method and lithium iron manganese phosphate battery

Abstract

The application discloses a kind of electrolyte for lithium manganese iron phosphate battery, it includes lithium salt, solvent and additive;The lithium salt includes lithium hexafluorophosphate, lithium difluoromethylsulfonylimide, lithium difluoro oxalate borate and lithium difluorophosphate at least one;The solvent includes vinyl carbonate, methyl ethyl carbonate, dimethyl carbonate and diethyl carbonate at least one;The additive includes vinylene carbonate, ethylene sulfate and N, N-dimethyl acrylamide DMAA at least one.In addition, the application also discloses a kind of lithium manganese iron phosphate battery and a kind of preparation method for electrolyte for lithium manganese iron phosphate battery.The application can improve the stability of positive electrode and electrolyte by using a kind of carbonate solvent and amide additive electrolyte formula for lithium manganese iron phosphate system, inhibit the oxidative decomposition of electrolyte, reduce the elution of transition metal, effectively improve the electrical performance of lithium manganese iron phosphate battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium manganese iron phosphate battery, and particularly relates to an electrolyte for lithium manganese iron phosphate battery, a preparation method thereof and a lithium manganese iron phosphate battery. BACKGROUND

[0002] Lithium manganese iron phosphate (LMFP) is a new type of high-energy positive electrode material with olivine structure, stable structure, high voltage platform and high energy density. However, the transition metal manganese in the LMFP material inevitably dissolves out during the charging and discharging process. The dissolution of manganese, on the one hand, affects the structure of the positive electrode material and the lithium storage capacity of the positive electrode; on the other hand, the dissolved manganese deposits on the negative electrode, affecting the diffusion of lithium ions, and at the same time, the deposited manganese catalyzes the decomposition of the electrolyte, leading to the consumption of active lithium and the formation of a thicker solid electrolyte interface film.

[0003] Therefore, it is necessary to improve the manganese dissolution problem of the LMFP material during the charging and discharging process. In the past few decades, researchers have taken various methods to improve the manganese dissolution problem, such as bulk doping, surface coating and electrolyte optimization.

[0004] Among the many methods to improve manganese dissolution, in terms of economy and effect, the optimization of electrolyte components is the most economical and effective. Electrolyte additives are the most important means to optimize electrolyte components and improve battery performance. They can form a thin and dense CEI film (i.e. electrolyte interface film) on the positive electrode during oxidation, preventing the electrolyte from undergoing oxidation reaction, and avoiding damage to the positive electrode material. In addition, the use of appropriate additives can also form a good SEI film (i.e. solid electrolyte interface film) on the negative electrode, preventing the electrolyte from reacting with the graphite negative electrode, and at the same time, a good SEI film can also inhibit the deposition of transition metals, which will also inhibit the manganese dissolution of the positive electrode material.

[0005] Currently, commercial electrolyte is mainly composed of lithium salt, solvent and additive. The most important lithium salt is LiPF6. The solvent mainly includes organic cyclic carbonate, linear carbonate and organic carboxylic acid ester. The organic cyclic carbonate includes ethylene carbonate (EC), propylene carbonate (PC) and fluoroethylene carbonate (FEC); the organic linear carbonate mainly includes methyl ethyl carbonate (EMC), diethyl carbonate (DEC) and dimethyl carbonate (DMC); among the organic carboxylic acid ester, propyl propionate (PP) and ethyl acetate (EA) are more common. Among them, the cyclic carbonate has strong polarity and strong solubility of lithium salt, but its viscosity is large and cannot be used alone. The linear ester has small polarity and low viscosity, which can improve the viscosity of the electrolyte. Therefore, the commonly used electrolyte is a mixture of cyclic carbonate and linear ester.

[0006] For the lithium manganese iron phosphate (LMFP) system, the main failure mode of the battery (i.e. the existing problem) is that: firstly, in the charge and discharge cycle, the dissolution of manganese in the positive electrode material will cause the reduction of lithium storage sites and the loss of positive active material. Secondly, the dissolved manganese is deposited on the negative electrode, which causes the destruction and regeneration of the SEI film, and a large amount of active lithium is consumed. SUMMARY

[0007] The purpose of the present application is to overcome the technical defects existing in the prior art, and provide an electrolyte for lithium manganese iron phosphate battery, a preparation method thereof and a lithium manganese iron phosphate battery.

[0008] To this end, the present application provides an electrolyte for lithium manganese iron phosphate battery, which comprises lithium salt, solvent and additive;

[0009] The lithium salt comprises at least one of lithium hexafluorophosphate, lithium bisfluoromethylsulfonylimide, lithium difluoro (oxalato) borate and lithium difluorophosphate;

[0010] The solvent comprises at least one of vinyl carbonate, methyl ethyl carbonate, dimethyl carbonate and diethyl carbonate;

[0011] The additive comprises at least one of vinylene carbonate, vinyl sulfate and N, N-dimethyl acrylamide DMAA;

[0012] The mass content of lithium salt in the total mass of electrolyte is 10% to 20%, the mass content of solvent in the total mass of electrolyte is 60% to 85%, and the mass content of additive in the total mass of electrolyte is 1.7% to 5%.

[0013] Preferably, when the lithium salt comprises lithium hexafluorophosphate, lithium bisfluoromethylsulfonylimide, lithium difluoro (oxalato) borate and lithium difluorophosphate, the mass content of lithium hexafluorophosphate in the total mass of electrolyte is 8% to 14%; the mass content of lithium bisfluoromethylsulfonylimide in the total mass of electrolyte is 1% to 2.5%, the mass content of lithium difluoro (oxalato) borate in the total mass of electrolyte is 0.5% to 2%, and the mass content of lithium difluorophosphate is 0.5% to 1.5%.

[0014] Preferably, when the solvent comprises vinyl carbonate, methyl ethyl carbonate, dimethyl carbonate and diethyl carbonate, the content of vinyl carbonate is 25% to 30% of the total mass of electrolyte; the content of methyl ethyl carbonate is 20% to 30% of the total mass of electrolyte; the content of diethyl carbonate is 10% to 15% of the total mass of electrolyte; and the content of dimethyl carbonate is 5% to 10% of the total mass of electrolyte.

[0015] Preferably, when the additive comprises the three additives of vinylene carbonate, vinyl sulfate and N,N-dimethyl acrylamide, the content of vinylene carbonate is 1-2% of the total mass of the electrolyte; the content of vinyl sulfate is 0.2-0.5% of the total mass of the electrolyte; and the content of N,N-dimethyl acrylamide is 0.5-2.5% of the total mass of the electrolyte.

[0016] Preferably, the first electrolyte formula is as follows:

[0017] The mass percentage content of each component in the electrolyte is as follows: lithium hexafluorophosphate 9.5-14%, lithium difluorooxalate borate 0.5%, vinylene carbonate 25-30%, methyl ethyl carbonate 25%-40%, dimethyl carbonate 10-15%, vinylene carbonate 0.7%-1%, and N,N-dimethyl acrylamide 1%.

[0018] Preferably, the second electrolyte formula is as follows:

[0019] The mass percentage content of each component in the electrolyte is as follows: lithium hexafluorophosphate 8.5-14%, lithium bisfluoromethylsulfonylimide 1-2.5%, lithium difluorooxalate borate 0.5-1.5%, vinylene carbonate 25-30%, methyl ethyl carbonate 25%-40%, diethyl carbonate 10-15%, vinylene carbonate 1%-2%, and N,N-dimethyl acrylamide 1.5%.

[0020] Preferably, the third electrolyte formula is as follows:

[0021] The mass percentage content of each component in the electrolyte is as follows: lithium hexafluorophosphate 8.5-14%, lithium bisfluoromethylsulfonylimide 1-2.5%, lithium difluorooxalate borate 0.5-1.5%, vinylene carbonate 25-30%, methyl ethyl carbonate 25%-40%, diethyl carbonate 10-15%, vinylene carbonate 1%-2%, vinyl sulfate 0.5%, and N,N-dimethyl acrylamide 2.5%.

[0022] In addition, the present application also provides a lithium iron manganese phosphate battery comprising the electrolyte for lithium iron manganese phosphate batteries as described above.

[0023] Furthermore, the present application also provides a preparation method of the electrolyte for lithium iron manganese phosphate batteries, which comprises the following steps:

[0024] In the first step, lithium salt, solvent and additives are pre-configured according to preset mass ratios;

[0025] In the second step, the lithium salt and additives are added to the solvent and mixed together;

[0026] In the third step, the mixture is stirred uniformly by a blender to obtain the electrolyte for lithium iron manganese phosphate batteries.

[0027] Preferably, in the first step, the lithium salt, including at least one of lithium hexafluorophosphate, lithium bisfluoromethylsulfonylimide, lithium difluoro(oxalato)borate and lithium difluorophosphate;

[0028] The solvent, including at least one of ethylene carbonate, methyl ethyl carbonate, dimethyl carbonate and diethyl carbonate;

[0029] The additive, including at least one of vinylene carbonate, vinyl sulfate and N,N-dimethylacrylamide DMAA;

[0030] The mass content of the lithium salt in the total mass of the electrolyte is 10% to 20%, the mass content of the solvent in the total mass of the electrolyte is 60% to 85%, and the mass content of the additive in the total mass of the electrolyte is 1.7% to 5%.

[0031] It can be seen from the technical solutions provided by the above application that, compared with the prior art, the application provides an electrolyte for a lithium iron manganese phosphate battery, a preparation method thereof and a lithium iron manganese phosphate battery, which are designed scientifically. The electrolyte formula of the carbonic acid ester solvent and the amide additive for the lithium iron manganese phosphate system can improve the stability of the positive electrode and the electrolyte, inhibit the oxidative decomposition of the electrolyte, reduce the dissolution of transition metals, and thus effectively improve the electrical performance of the lithium iron manganese phosphate battery, which has great practical significance. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 A flowchart of a preparation method of the electrolyte for the lithium iron manganese phosphate battery provided by the application is shown. DETAILED DESCRIPTION

[0033] In order to enable personnel in the technical field to better understand the application solutions, the application is further described in detail below in combination with the drawings and embodiments.

[0034] The application provides an electrolyte for a lithium iron manganese phosphate battery, which comprises a lithium salt, a solvent and an additive;

[0035] The lithium salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium bisfluoromethylsulfonylimide (LiFSi), lithium difluoro(oxalato)borate (LiDFOB) and lithium difluorophosphate (LiPO2F2);

[0036] The solvent includes at least one of ethylene carbonate (EC), methyl ethyl carbonate (EMC), dimethyl carbonate (DMC) and diethyl carbonate (DEC);

[0037] The additive includes at least one of vinylene carbonate (VC), vinyl sulfate (DTD) and N,N-dimethylacrylamide (DMAA).

[0038] In the present application, specifically, in the electrolyte formula, the mass content (i.e. mass percentage) of lithium salt in the total mass of electrolyte is 10% to 20%, the mass content of solvent in the total mass of electrolyte is 60% to 85%, and the mass content of additive in the total mass of electrolyte is 1.7% to 5%.

[0039] In the present application, specifically, in the electrolyte formula, when the lithium salt includes lithium hexafluorophosphate (LiPF6), lithium bisfluoromethylsulfonylimide (LiFSi), lithium difluoro(oxalato)borate (LiDFOB) and lithium difluorophosphate (LiPO2F2), the mass content of lithium hexafluorophosphate (LiPF6) in the total mass of electrolyte is 8% to 14%; the mass content of lithium bisfluoromethylsulfonylimide (LiFSi) in the total mass of electrolyte is 1% to 2.5%; the mass content of lithium difluoro(oxalato)borate (LiDFOB) in the total mass of electrolyte is 0.5% to 2%; and the mass content of lithium difluorophosphate (LiPO2F2) in the total mass of electrolyte is 0.5% to 1.5%.

[0040] In the present application, specifically, in the electrolyte formula, when the solvent includes vinyl carbonate (EC), methyl ethyl carbonate (EMC), dimethyl carbonate (DMC) and diethyl carbonate (DEC), the content of vinyl carbonate (EC) is 25% to 30% of the total mass of electrolyte; the content of methyl ethyl carbonate (EMC) is 20% to 30% of the total mass of electrolyte; the content of diethyl carbonate (DEC) is 10% to 15% of the total mass of electrolyte; and the content of dimethyl carbonate (DMC) is 5% to 10% of the total mass of electrolyte.

[0041] In the present application, specifically, in the electrolyte formula, when the additive includes vinylene carbonate (VC), vinyl sulfate (DTD) and N,N-dimethylacrylamide (DMAA), the content of vinylene carbonate (VC) is 1% to 2% of the total mass of electrolyte; the content of vinyl sulfate (DTD) is 0.2% to 0.5% of the total mass of electrolyte; and the content of N,N-dimethylacrylamide (DMAA) is 0.5% to 2.5% of the total mass of electrolyte.

[0042] In the present application, specifically, for the electrolyte for lithium iron manganese phosphate battery, the first electrolyte formula contained therein can be as follows:

[0043] The mass percentage content of each component in the electrolyte is respectively: lithium hexafluorophosphate (LiPF6) 9.5-14%, lithium difluoro(oxalato)borate (LiDFOB) 0.5%, ethylene carbonate (EC) 25-30%, methyl ethyl carbonate (EMC) 25%-40%, dimethyl carbonate (DMC) 10-15%, vinylene carbonate (VC) 0.7%-1%, N,N-dimethyl acrylamide (DMAA) 1%;

[0044] In the present application, the second electrolyte formula can be as follows:

[0045] The mass percentage content of each component in the electrolyte is respectively: lithium hexafluorophosphate (LiPF6) 8.5-14%, lithium bis(fluorosulfonyl)imide (LiFSi) 1-2.5%, lithium difluoro(oxalato)borate (LiDFOB) 0.5-1.5%, ethylene carbonate (EC) 25-30%, methyl ethyl carbonate (EMC) 25%-40%, diethyl carbonate (DEC) 10-15%, vinylene carbonate (VC) 1%-2%, N,N-dimethyl acrylamide (DMAA) 1.5%;

[0046] In the present application, the third electrolyte formula can be as follows:

[0047] The mass percentage content of each component in the electrolyte is respectively: lithium hexafluorophosphate (LiPF6) 8.5-14%, lithium bis(fluorosulfonyl)imide (LiFSi) 1-2.5%, lithium difluoro(oxalato)borate (LiDFOB) 0.5-1.5%, ethylene carbonate (EC) 25-30%, methyl ethyl carbonate (EMC) 25%-40%, diethyl carbonate (DEC) 10-15%, vinylene carbonate (VC) 1%-2%, vinyl sulfite (DTD) 0.5%, N,N-dimethyl acrylamide (DMAA) 2.5%;

[0048] It should be noted that, for the present application, the use of DMAA additive effectively improves the transition metal elution problem of lithium manganese iron phosphate battery. By using the amide additive, the interface stability between the lithium manganese iron phosphate positive electrode and the electrolyte can be improved, the side reaction of the electrolyte can be reduced, the consumption of active lithium can be reduced, and the battery performance can be improved. This scheme does not require additional process design in the process and is easy to implement.

[0049] It should be noted that DMAA has a higher highest occupied molecular orbital (HOMO) energy than the solvent molecules of the electrolyte, which can oxidize on the positive side in preference to the solvent, avoiding the oxidation and decomposition of the electrolyte; at the same time, DMAA has a lower lowest unoccupied molecular orbital energy than the solvent molecules, which can reduce on the negative side in preference to the solvent, and the generated reduction product plays a passivation role on the negative electrode, avoiding the reaction of the solvent. In summary, DMAA can simultaneously undergo oxidation and reduction reactions on the positive and negative electrodes, inhibit the side reactions of the electrolyte on the positive and negative electrodes, and thus reduce the consumption of active lithium.

[0050] In addition, for the present application, the present application also provides a lithium iron manganese phosphate battery comprising the aforementioned electrolyte for lithium iron manganese phosphate battery.

[0051] Referring to Figure 1 In order to prepare the aforementioned electrolyte for lithium iron manganese phosphate battery, the present application provides a preparation method of the electrolyte for lithium iron manganese phosphate battery, which specifically comprises the following steps:

[0052] In the first step, lithium salt, solvent and additive are pre-configured according to the preset mass ratio;

[0053] In the second step, the lithium salt and the additive are added to the solvent and mixed together;

[0054] In the third step, the electrolyte for lithium iron manganese phosphate battery is finally configured by fully stirring and uniformizing through a stirrer.

[0055] In the present application, in the first step, the lithium salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium bisfluoromethylsulfonylimide (LiFSi), lithium difluoro oxalate borate (LiDFOB) and lithium difluorophosphate (LiPO2F2);

[0056] In the present application, in the first step, the solvent includes at least one of ethylene carbonate (EC), methyl ethyl carbonate (EMC), dimethyl carbonate (DMC) and diethyl carbonate (DEC);

[0057] In the present application, in the first step, the additive includes at least one of vinylene carbonate (VC), vinyl sulfate (DTD) and N,N-dimethyl acrylamide (DMAA).

[0058] In order to more clearly understand the technical solutions of the present application, the technical solutions of the present application will be described below through specific embodiments.

[0059] Embodiment 1

[0060] In the embodiment 1 of the present application, the mass percentage content of each component in the electrolyte is respectively: lithium hexafluorophosphate (LiPF6) 13.2%, lithium difluoro(oxalato)borate (LiDFOB) 0.8%, ethylene carbonate (EC) 30%, ethyl methyl carbonate (EMC) 43%, diethyl carbonate (DEC) 10%, vinylene carbonate (VC) 2%; vinyl sulfate (DTD) 1.2%, N,N-dimethylacrylamide (DMAA) 0.5%.

[0061] Embodiment 2

[0062] In the embodiment 2 of the present application, the mass percentage content of each component in the electrolyte is respectively: lithium hexafluorophosphate (LiPF6) 13.2%, lithium difluoro(oxalato)borate (LiDFOB) 0.8%, ethylene carbonate (EC) 30%, ethyl methyl carbonate (EMC) 42.5%, diethyl carbonate (DEC) 10%, vinylene carbonate (VC) 2%; vinyl sulfate (DTD) 0.7%, N,N-dimethylacrylamide (DMAA) 1%.

[0063] Embodiment 3

[0064] In the embodiment 1 of the present application, the mass percentage content of each component in the electrolyte is respectively: lithium hexafluorophosphate (LiPF6) 13.2%, lithium difluoro(oxalato)borate (LiDFOB) 0.8%, ethylene carbonate (EC) 30%, ethyl methyl carbonate (EMC) 41%, diethyl carbonate (DEC) 10%, vinylene carbonate (VC) 2%; vinyl sulfate (DTD) 0.5%, N,N-dimethylacrylamide (DMAA) 2.5%.

[0065] Embodiment 4

[0066] In the embodiment 4 of the present application, the mass percentage content of each component in the electrolyte is respectively: lithium hexafluorophosphate (LiPF6) 11%, lithium bis(fluorosulfonyl)imide (LiFSi) 2.2%, lithium difluoro(oxalato)borate (LiDFOB) 0.8%, ethylene carbonate (EC) 30%, ethyl methyl carbonate (EMC) 42.5%, diethyl carbonate (DEC) 10%, vinylene carbonate (VC) 2%, vinyl sulfate (DTD) 0.5%, N,N-dimethylacrylamide (DMAA) 1%;

[0067] Comparative Example 1

[0068] An existing formula of electrolyte, the mass percentage content of each component in the electrolyte is: lithium hexafluorophosphate (LiPF6) 13.2%, lithium difluoro oxalate borate (LiDFOB) 0.8%, ethylene carbonate (EC) 30%, methyl ethyl carbonate (EMC) 41%, diethyl carbonate (DEC) 10%, vinylene carbonate (VC) 2%, vinyl sulfate (DTD) 0.5%.

[0069] Comparative Example 2

[0070] An existing formula of electrolyte, the mass percentage content of each component in the electrolyte is: lithium hexafluorophosphate (LiPF6) 13.2%, lithium difluoro oxalate borate (LiDFOB) 0.8%, ethylene carbonate (EC) 30%, methyl ethyl carbonate (EMC) 41%, diethyl carbonate (DEC) 10%, vinylene carbonate (VC) 2%, vinyl sulfate (DTD) 0.5%.

[0071] Based on the electrolytes of Examples 1-4 above and the electrolytes of Comparative Examples 1-2, further lithium manganese iron phosphate (LMFP) batteries are prepared. The specific preparation method is as shown below (except that the formula of the electrolyte is different, the others are the same):

[0072] First step, preparation of LMFP positive electrode sheet;

[0073] LMFP, conductive agent CNT, conductive agent super P and binder PVDF are uniformly dispersed in NMP solvent at a mass ratio of 96:0.5:1.0:2.5. After stirring in a vacuum environment for a period of time, the solid content of the positive electrode slurry is controlled at 60% out of the discharge, the positive electrode slurry is uniformly coated on the two surfaces of the carbon-coated aluminum foil, the electrode sheet is dried, and the electrode sheet is pressed to 2.4g / cm 3 The compaction is rolled, and finally cut into a specified size for use.

[0074] Second step, preparation of negative electrode sheet;

[0075] Graphite, super P, binder CMC and binder SBR are uniformly dispersed in deionized water at a mass ratio of 96.3:1.0:1.2:1.5. After sufficient stirring in a vacuum, a uniformly mixed negative electrode slurry is obtained, the solid content of the negative electrode slurry is controlled at 50% out of the discharge, the negative electrode slurry is uniformly coated on the two surfaces of the negative electrode current collector copper foil, the electrode sheet is dried, and the electrode sheet is pressed to 1.47g / cm 3 The compaction is rolled. Finally cut into a specified size for use.

[0076] Third step, preparation of separator;

[0077] The electrolyte is prepared according to the fourth step. The electrolyte used is the electrolyte described in Examples 1-4 or Comparative Examples 1-2.

[0078] Fourth step, preparation of electrolyte;

[0079] The electrolyte used is the electrolyte described in Examples 1-4 or Comparative Examples 1-2.

[0080] Fifth step, preparation of battery.

[0081] The above positive electrode sheet, separator and negative electrode sheet are stacked by the lamination process to form a battery electrode group with 9 positive electrode sheets and 10 negative electrode sheets. After heat pressing and tab welding of the battery electrode group, the battery is loaded into a battery case. After drying the battery, the electrolyte prepared in the fourth step is injected, and after standing at room temperature, vacuum exhaust, heat sealing, formation and sorting, the finished battery is obtained.

[0082] The performance test of the battery prepared by the electrolyte of Examples 1-4 and Comparative Examples 1-2 is as follows.

[0083] Cycle performance test: the battery prepared in Examples 1-4 and Comparative Examples 1-2 is charged at 1C rate in a temperature chamber at 45°C (high temperature cycle), when the battery voltage reaches 4.2V, it is converted to constant voltage charging until the current decreases to 0.05C, then it is discharged at 1C rate until the battery voltage decreases to 2.5V, and then it is rested for 10 minutes. Repeat the above charging and discharging steps until the capacity of the battery decays to 80% of the initial capacity, and record the cycle number of the battery.

[0084] The test results of the battery prepared by the electrolyte of Examples 1-4 and Comparative Examples 1-2 are shown in Table 1 below.

[0085] Table 1: Performance test results of batteries of each example and comparative example.

[0086]

[0087] From the above Examples 1, 2 and 3, it can be seen that when the addition amount of DMAA is 1%, the 25°C cycle and 45°C cycle performance of the battery is the best, with room temperature cycle reaching 3200 times and high temperature cycle reaching 2300 times.

[0088] From the comparison of Example 1 and Comparative Example 1, it can be seen that the addition of DMAA can significantly improve the room temperature and high temperature cycle performance of the battery.

[0089] By comparing example 4 with comparative example 2, it can be seen that when lithium hexafluorophosphate (LiPF6) and lithium bisfluoromethylsulfonylimide (LiFSi) are used as lithium salts, DMAA can still improve the room temperature cycle performance and high temperature cycle performance of the battery.

[0090] Compared with the prior art, the electrolyte for a lithium iron manganese phosphate battery, the preparation method and the lithium iron manganese phosphate battery have the following remarkable technical effects:

[0091] 1. The electrolyte of the application is composed of carbonate solvents, lithium salts and amide additives, which effectively improves the transition metal dissolution and the compatibility of the positive electrode and the electrolyte, and improves the battery performance.

[0092] 2. In view of the lithium iron manganese phosphate battery in the repeated charging and discharging process, on the one hand, the dissolution of manganese causes the decrease of the positive electrode lithium storage capacity and the loss of the positive electrode active material; on the other hand, the dissolved manganese is deposited on the negative electrode, which destroys the SEI structure of the negative electrode and causes the consumption of a large amount of active lithium in the repeated repair of SEI. The electrolyte system of the application can form a dense and uniform protective layer on the positive electrode by using carbonate solvents and amide additives, which can avoid the damage of the electrolyte to the positive electrode material and improve the electrical performance of the lithium iron manganese phosphate battery.

[0093] In summary, compared with the prior art, the electrolyte for a lithium iron manganese phosphate battery, the preparation method and the lithium iron manganese phosphate battery provided by the application have the following remarkable technical effects: the design is scientific, the electrolyte formula of the application for the lithium iron manganese phosphate system can improve the stability of the positive electrode and the electrolyte, inhibit the oxidative decomposition of the electrolyte, reduce the dissolution of transition metals, and finally effectively improve the electrical performance of the lithium iron manganese phosphate battery, which has great practical significance.

[0094] It should be noted that the use of DMAA additive has a higher highest occupied molecular orbital (HOMO) energy than the electrolyte solvent molecules, so the oxidation reaction can occur preferentially, and the generated oxidation products adhere to the surface of the positive electrode, so the oxidation of the electrolyte can inhibit the oxidative decomposition of the electrolyte, and also inhibit the dissolution of transition metals.

[0095] The above description is only the preferred embodiment of the application, and it should be noted that for ordinary skilled persons in the art, without departing from the principle of the application, a number of improvements and refinements can be made, which should also be considered as the protection scope of the application.

Claims

1. An electrolyte for a lithium iron manganese phosphate battery, characterized in that, It includes lithium salt, solvent and additive; The lithium salt includes at least one of lithium hexafluorophosphate, lithium bisfluoromethylsulfonylimide, lithium difluoro(oxalato)borate and lithium difluorophosphate; The solvent includes at least one of vinyl carbonate, ethyl methyl carbonate, dimethyl carbonate and diethyl carbonate; The additive includes vinylene carbonate, vinyl sulfate and N,N-dimethyl acrylamide, the content of vinylene carbonate is 1-2% of the total mass of the electrolyte; the content of vinyl sulfate is 0.2-0.5% of the total mass of the electrolyte; the content of N,N-dimethyl acrylamide is 0.5-2.5% of the total mass of the electrolyte; The mass content of lithium salt in the total mass of the electrolyte is 10-20%, the mass content of solvent in the total mass of the electrolyte is 60-85%, and the mass content of additive in the total mass of the electrolyte is 1.7-5%.

2. The electrolyte for lithium iron manganese phosphate battery as claimed in claim 1, wherein, The lithium salt includes lithium hexafluorophosphate, lithium bisfluoromethylsulfonylimide, lithium difluoro(oxalato)borate and lithium difluorophosphate, the mass content of lithium hexafluorophosphate in the total mass of the electrolyte is 8-14%; the mass content of lithium bisfluoromethylsulfonylimide in the total mass of the electrolyte is 1-2.5%; the mass content of lithium difluoro(oxalato)borate in the total mass of the electrolyte is 0.5-2%; and the mass content of lithium difluorophosphate is 0.5-1.5%.

3. The electrolyte for lithium iron manganese phosphate battery as claimed in claim 1, wherein, The solvent includes vinyl carbonate, ethyl methyl carbonate, dimethyl carbonate and diethyl carbonate, the content of vinyl carbonate is 25-30% of the total mass of the electrolyte; the content of ethyl methyl carbonate is 20-30% of the total mass of the electrolyte; the content of diethyl carbonate is 10-15% of the total mass of the electrolyte; and the content of dimethyl carbonate is 5-10% of the total mass of the electrolyte. ​ 4. The electrolyte for lithium iron manganese phosphate battery as claimed in claim 1, wherein, The third electrolyte formula is as follows: ​ The mass percentage content of each component in the electrolyte is as follows: lithium hexafluorophosphate 8.5-14%, lithium bisfluoromethylsulfonylimide 2.5%, lithium difluoro(oxalato)borate 0.5-1.5%, vinyl carbonate 25-30%, ethyl methyl carbonate 25-40%, diethyl carbonate 10-15%, vinylene carbonate 1%, vinyl sulfate 0.5% and N,N-dimethyl acrylamide 2.5%.

5. A lithium iron manganese phosphate battery, characterized by, The electrolyte for lithium iron manganese phosphate battery according to any one of claims 1-4.

6. A method for preparing an electrolyte for a lithium manganese iron phosphate battery according to any one of claims 1 to 4, characterized in that: The method comprises the following steps: In the first step, the lithium salt, the solvent and the additive are pre-configured according to the preset mass ratio; In the second step, the lithium salt and the additive are added to the solvent and mixed together; In the third step, the mixture is stirred uniformly by a stirrer to obtain the electrolyte for lithium iron manganese phosphate battery.

7. The method of claim 6, wherein the electrolyte is prepared by adding the electrolyte components to the solvent and stirring the mixture at a temperature of 20 to 30°C for 1 to 3 hours. In the first step, the lithium salt includes at least one of lithium hexafluorophosphate, lithium bisfluoromethylsulfonylimide, lithium difluoro(oxalato)borate and lithium difluorophosphate; The solvent includes at least one of vinyl carbonate, ethyl methyl carbonate, dimethyl carbonate and diethyl carbonate; The additive comprises three kinds of additives of vinylene carbonate, vinyl sulfate and N,N-dimethyl acrylamide, the content of the vinylene carbonate accounts for 1-2% of the total mass of the electrolyte; the content of the vinyl sulfate accounts for 0.2-0.5% of the total mass of the electrolyte; the content of the N,N-dimethyl acrylamide accounts for 0.5-2.5% of the total mass of the electrolyte; The mass content of the lithium salt accounts for 10-20% of the total mass of the electrolyte, the mass content of the solvent accounts for 60-85% of the total mass of the electrolyte, and the mass content of the additive accounts for 1.7-5% of the total mass of the electrolyte.

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