Lithium manganese iron phosphate battery electrolyte additive, lithium manganese iron phosphate battery electrolyte as well as preparation method and application of lithium manganese iron phosphate battery electrolyte

Abstract

The invention relates to the technical field of lithium ion batteries, in particular to a lithium iron manganese phosphate battery electrolyte additive, a lithium iron manganese phosphate battery electrolyte and a preparation method and application thereof. The lithium iron manganese phosphate battery electrolyte additive is composed of an additive A and an additive B. The additive A is a borate compound, and an electrode structure is prevented from collapsing by capturing dissolved Mn and forming a compact catholyte interface film. And the additive B is a silane derivative containing a Si-O bond, can remove free HF in the electrolyte, participates in construction of a catholyte interface film, and reduces interface impedance and lithium ion transmission resistance. The lithium iron manganese phosphate battery electrolyte additive effectively inhibits Mn dissolution and removes HF, solves the problems of fast capacity fading and poor circulation caused by manganese dissolution of a lithium iron manganese phosphate battery, and endows the lithium iron manganese phosphate battery with high-rate charge-discharge capability and long cycle life.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and in particular to an electrolyte additive for a lithium iron manganese phosphate battery, an electrolyte for a lithium iron manganese phosphate battery, and a preparation method and application thereof. Background Art

[0002] Cathode materials play a central role in the energy capacity and cost-effectiveness of lithium-ion batteries. LiFePO4 has become the most promising cathode material for lithium-ion batteries due to its low price and good cycle stability. However, the low operating voltage of LiFePO4 leads to its low energy density, which limits its further development in high-energy power batteries. Researchers partially replaced the Fe in LiFePO4 with Mn to obtain LiFe 1-x Mn x PO4 is considered to be a promising next-generation cathode material due to its balance between energy density and safety.

[0003] However, on the one hand, Mn 3+ The Jahn-Teller effect causes the lithium manganese iron phosphate cathode to dissolve during the electrochemical cycle, resulting in the collapse of the cathode crystal structure; under the action of the electric field, Mn is deposited on the electrode surface to form inactive materials, destroying the electrode-electrolyte interface, causing irreversible capacity loss, and seriously affecting the cycle stability of lithium-ion batteries. On the other hand, traditional carbonate electrolytes oxidize and decompose at voltages greater than 4.1V, producing H + This also forms hydrofluoric acid, which causes corrosion at the electrode-electrolyte interface and accelerates the dissolution of transition metals in the lithium iron manganese phosphate. These issues can lead to increased impedance and a rapid decrease in rate performance in lithium-ion batteries, significantly impacting their cycle life. Considering these factors, establishing a stable cathode-electrolyte interface (CEI) and suppressing HF generation are crucial to ensuring a long cycle life for lithium iron manganese phosphate cathodes at high voltages.

[0004] To address this issue, interface modification is one of the most promising methods to inhibit Mn dissolution. The conventional solution is to add a coating material to the lithium manganese iron phosphate cathode, but coating materials often have problems such as poor conductivity, poor lithium ion diffusion, and uneven surface coating. Summary of the Invention

[0005] Based on the deficiencies in the above-mentioned prior art, the present invention provides an electrolyte additive for lithium iron manganese phosphate batteries, an electrolyte for lithium iron manganese phosphate batteries, and a preparation method and application thereof. The electrolyte additive for lithium iron manganese phosphate batteries is composed of additive A and additive B. Additive A is a borate compound that prevents electrode structure collapse by capturing dissolved Mn and forming a dense cathode electrolyte interface film; additive B is a silane derivative containing Si-O bonds that can remove free HF in the electrolyte and participate in the construction of the cathode electrolyte interface film, reducing interfacial impedance and lithium ion transmission resistance. The electrolyte additive for lithium iron manganese phosphate batteries of the present invention effectively inhibits Mn dissolution and removes HF, inhibits battery side reactions, solves the problems of rapid capacity decay and poor cycle life of lithium iron manganese phosphate batteries caused by manganese dissolution, and gives lithium iron manganese phosphate batteries high-rate charge and discharge capabilities and long cycle life.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions: The first purpose of the present invention is to provide an electrolyte additive for lithium manganese iron phosphate batteries, aiming to solve the problem of battery performance degradation caused by the dissolution of manganese ions. The lithium manganese iron phosphate positive electrode material has multiple defects caused by the dissolution of Mn: 2+ Dissolved in the electrolyte, Mn 3+ The Jan-Taylor effect aggravates the lattice distortion and dissolution. The dissolved manganese migrates to the negative electrode to catalyze the decomposition of the SEI film and destroy the thermal stability of the electrolyte. At the same time, the side reactions generate inert substances that reduce the activity of the electrode material.

[0007] The second object of the present invention is to provide an electrolyte for lithium manganese iron phosphate batteries, which has the advantages of achieving excellent rate performance and long cycle stability of the battery cell at high voltage.

[0008] The third object of the present invention is to provide a method for preparing lithium manganese iron phosphate battery electrolyte, which is compatible with existing production line equipment and can achieve efficient and low-cost large-scale production without complex modifications to meet mass production needs.

[0009] A fourth object of the present invention is to provide a lithium manganese iron phosphate battery, which is assembled using a lithium manganese iron phosphate battery electrolyte.

[0010] The present invention protects an electrolyte additive for lithium manganese iron phosphate batteries, which comprises an additive A and an additive B. Additive A is a borate compound, and its structural formula is , wherein R1, R2, R3, and R4 are independently selected from one of pentafluorophenyl, oxalate, and F; Additive B is selected from at least one of trimethylsiloxane phosphate, trimethylsiloxane carbonate, trimethylsilyl chloride, trimethylsilyl isocyanate, trimethylsilyl borate, trimethylsiloxane, hexamethyldisiloxane, trimethylsilyl glycol dimethyl ether, trimethyltrifluoropropylsilane, trimethylsilyl perfluoropolyether, and trimethylsilyl cyanide.

[0011] Preferably, the mass ratio of additive A to additive B is 1-5:1; further, the mass ratio of additive A to additive B is 1-3:1.

[0012] The present invention protects a lithium manganese iron phosphate battery electrolyte, which is composed of the above-mentioned lithium manganese iron phosphate battery electrolyte additive, a lithium salt and an organic solvent.

[0013] Preferably, the mass percentage of additive A in the lithium manganese iron phosphate battery electrolyte is 0.5wt%~2.5wt%.

[0014] Preferably, the mass percentage of additive A in the lithium manganese iron phosphate battery electrolyte is 1.5 wt%.

[0015] Preferably, the mass percentage of additive B in the lithium manganese iron phosphate battery electrolyte is 0.5wt%~1.5wt%.

[0016] Preferably, the mass percentage of the lithium manganese iron phosphate battery electrolyte additive in the lithium manganese iron phosphate battery electrolyte is 1wt%~3wt%.

[0017] Preferably, the organic solvent is a carbonate solvent, and the mass percentage of the carbonate solvent in the lithium iron manganese phosphate battery electrolyte is 65wt% to 85wt%. The carbonate solvent is selected from ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), or ethyl propyl carbonate (EPC).

[0018] Preferably, the mass percentage of the lithium salt in the lithium iron manganese phosphate battery electrolyte is 10wt% to 20wt%. The lithium salt is selected from one or more of lithium hexafluorophosphate (LiPF6), lithium difluorooxalatoborate (LiDFOB), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl imide) (LiTFSI), lithium difluorophosphate (LiPO2F2), lithium bis(oxalatoborate) (LiBOB), lithium bis(fluorosulfonyl imide) (LiFSI), and lithium trifluoromethanesulfonate (LiSO3CF3). According to the embodiments of the present invention, the lithium salt of the present invention is not limited thereto, and all existing lithium salts for lithium-ion batteries are suitable for the present invention.

[0019] The present invention also provides a method for preparing a lithium iron manganese phosphate battery electrolyte, which comprises the following steps: mixing an organic solvent, a lithium salt and a lithium iron manganese phosphate battery electrolyte additive, and stirring the mixture uniformly to obtain the lithium iron manganese phosphate battery electrolyte.

[0020] The present invention also protects the use of lithium iron manganese phosphate battery electrolyte in the preparation of lithium iron manganese phosphate battery. The lithium iron manganese phosphate battery consists of a lithium iron manganese phosphate positive electrode sheet, a negative electrode sheet, a separator and the lithium iron manganese phosphate battery electrolyte.

[0021] Compared with the prior art, the present invention has the following beneficial effects: 1. The lithium iron manganese phosphate battery electrolyte additive of the present invention consists of additive A and additive B. Additive A can anchor manganese ions through Lewis acid properties, inhibit the dissolution of manganese ions and the release of lattice oxygen, and form a stable CEI interface film, thereby stabilizing the structure of the lithium iron manganese phosphate positive electrode and inhibiting the decomposition of the carbonate electrolyte; Additive B contains a silicon functional group and has high reactivity with HF, which removes HF in the lithium iron manganese phosphate battery electrolyte, thereby protecting the lithium iron manganese phosphate interface and further inhibiting the dissolution of manganese ions. The two additives synergistically improve the kinetic performance of the lithium iron manganese phosphate battery, significantly improve the cycle life and rate performance of the lithium iron manganese phosphate battery in the range of 2.0V to 4.5V, and ultimately achieve excellent rate performance and long cycle stability of the battery cell at high voltage.

[0022] 2. The present invention provides an electrolyte for lithium manganese iron phosphate batteries composed of specific components. By assembling button batteries and comparing the test results of battery capacity retention and long cycle life with different additives, the corresponding performance optimization effect under the optimal addition ratio is obtained, and the best lithium manganese iron phosphate battery is obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The graphs are for the cycle performance of lithium-ion batteries assembled using the lithium manganese iron phosphate battery electrolyte of Example 2 and the electrolyte of Comparative Example 8, respectively.

[0024] Figure 2 The iron and manganese elements are in the negative electrode metal lithium of the lithium ion battery assembled using the lithium manganese iron phosphate battery electrolyte of Example 2 and the electrolyte of Comparative Example 8 respectively. DETAILED DESCRIPTION

[0025] The technical solutions of the present invention are described in detail below with reference to specific embodiments and accompanying drawings. It should be noted that the examples listed (including the examples shown in the accompanying drawings) are for illustrative purposes only, intended to clearly illustrate the implementation methods and effects of the present invention, and should not be construed as limiting the scope of protection of this application. Unless otherwise specified, the raw materials, reagents, preparation methods, etc. used in the examples are conventional technical means in the art, and their specific parameters and steps can be adjusted according to actual needs. It should be further clarified that the descriptive content of the above embodiments does not in any way limit the scope of the claims of the present invention. Reasonable substitutions or improvements made by those skilled in the art within the framework of the claims are all within the scope of protection of the present invention.

[0026] Taking into account the technical defects of the existing technology of adding coating materials to the lithium manganese iron phosphate positive electrode, the present invention starts from the modification of the electrolyte and provides a lithium manganese iron phosphate battery electrolyte additive composed of additive A and additive B. Additive A is a borate compound, which prevents the collapse of the electrode structure by capturing the dissolved Mn and forming a dense cathode electrolyte interface film; Additive B is a silane derivative containing Si-O bonds, which can remove free HF in the electrolyte and participate in the construction of the cathode electrolyte interface film, reducing the interface impedance and lithium ion transmission resistance.

[0027] The following examples and comparative examples are used to further study the technical solution of the present invention. The specific research methods and results are as follows: Example 1 A method for preparing a lithium manganese iron phosphate battery electrolyte comprises the following steps: In a high-purity argon glove box with an oxygen content and a water content of no more than 0.01 ppm, dimethyl carbonate (DMC) and ethylene carbonate (EC) were mixed in a volume ratio of 1:1 to obtain a mixed solvent, which was then dehydrated using a 4Å molecular sieve. Lithium hexafluorophosphate (LiPF6) was added to a concentration of 1 mol / L. Dry ice was used to control the temperature during the stirring and dissolving process to ensure that the temperature fluctuation did not exceed 2°C. After the lithium hexafluorophosphate was completely dissolved, 1.0% by mass of additive A and 0.5% by mass of additive B were added to the lithium manganese iron phosphate battery electrolyte, and the mixture was thoroughly stirred until uniformly mixed to obtain a colorless and transparent lithium manganese iron phosphate battery electrolyte.

[0028] Example 2 A method for preparing a lithium manganese iron phosphate battery electrolyte comprises the following steps: In a high-purity argon glove box with an oxygen content and a water content not higher than 0.01 ppm, dimethyl carbonate (DMC) and ethylene carbonate (EC) were mixed in a volume ratio of 1:1 to obtain a mixed solvent, which was then dehydrated using a 4Å molecular sieve. Lithium hexafluorophosphate (LiPF6) was added to a concentration of 1 mol / L. Dry ice was used to control the temperature during the stirring and dissolving process to ensure that the temperature fluctuation did not exceed 2°C. After the lithium hexafluorophosphate was completely dissolved, 1.5% by mass of additive A and 0.5% by mass of additive B were added to the lithium manganese iron phosphate battery electrolyte, and the mixture was thoroughly stirred until uniformly mixed to obtain a colorless and transparent lithium manganese iron phosphate battery electrolyte.

[0029] Example 3 A method for preparing a lithium manganese iron phosphate battery electrolyte comprises the following steps: In a high-purity argon glove box with an oxygen content and a water content not exceeding 0.01 ppm, dimethyl carbonate (DMC) and ethylene carbonate (EC) were mixed in a volume ratio of 1:1 to obtain a mixed solvent, which was then dehydrated using a 4Å molecular sieve. Lithium hexafluorophosphate (LiPF6) was added to a concentration of 1 mol / L. Dry ice was used to control the temperature during the stirring and dissolving process to ensure that the temperature fluctuation did not exceed 2°C. After the lithium hexafluorophosphate was completely dissolved, 0.5% by mass of additive A and 0.5% by mass of additive B were added to the lithium manganese iron phosphate battery electrolyte, and the mixture was thoroughly stirred until uniformly mixed to obtain a colorless and transparent lithium manganese iron phosphate battery electrolyte.

[0030] Example 4 A method for preparing a lithium manganese iron phosphate battery electrolyte comprises the following steps: In a high-purity argon glove box with an oxygen content and a water content not higher than 0.01 ppm, dimethyl carbonate (DMC) and ethylene carbonate (EC) were mixed in a volume ratio of 1:1 to obtain a mixed solvent, which was then dehydrated using a 4Å molecular sieve. Lithium hexafluorophosphate (LiPF6) was added to a concentration of 1 mol / L. Dry ice was used to control the temperature during the stirring and dissolving process to ensure that the temperature fluctuation did not exceed 2°C. After the lithium hexafluorophosphate was completely dissolved, 2.5% by mass of additive A and 0.5% by mass of additive B were added to the lithium manganese iron phosphate battery electrolyte, and the mixture was thoroughly stirred until uniformly mixed to obtain a colorless and transparent lithium manganese iron phosphate battery electrolyte.

[0031] Comparative Example 1 A method for preparing a lithium manganese iron phosphate battery electrolyte comprises the following steps: In a high-purity argon glove box with an oxygen content and a water content of no more than 0.01 ppm, dimethyl carbonate (DMC) and ethylene carbonate (EC) were mixed in a volume ratio of 1:1 to obtain a mixed solvent. The mixed solvent was then dehydrated using a 4Å molecular sieve, and lithium hexafluorophosphate (LiPF6) was added to a concentration of 1 mol / L. Dry ice was used to control the temperature during the stirring and dissolving process to ensure that the temperature fluctuation did not exceed 2°C. After the lithium hexafluorophosphate was completely dissolved, 0.5% by mass of additive A was added to the lithium manganese iron phosphate battery electrolyte, and the mixture was thoroughly stirred until the mixture was uniformly mixed to obtain a colorless and transparent lithium manganese iron phosphate battery electrolyte.

[0032] Comparative Example 2 A method for preparing a lithium manganese iron phosphate battery electrolyte comprises the following steps: In a high-purity argon glove box with an oxygen content and a water content of no more than 0.01 ppm, dimethyl carbonate (DMC) and ethylene carbonate (EC) were mixed in a volume ratio of 1:1 to obtain a mixed solvent. The mixed solvent was then dehydrated using a 4Å molecular sieve, and lithium hexafluorophosphate (LiPF6) was added to a concentration of 1 mol / L. Dry ice was used to control the temperature during the stirring and dissolving process to ensure that the temperature fluctuation did not exceed 2°C. After the lithium hexafluorophosphate was completely dissolved, 1.0% by mass of additive A was added to the lithium manganese iron phosphate battery electrolyte, and the mixture was thoroughly stirred until the mixture was uniformly mixed to obtain a colorless and transparent lithium manganese iron phosphate battery electrolyte.

[0033] Comparative Example 3 A method for preparing a lithium manganese iron phosphate battery electrolyte comprises the following steps: In a high-purity argon glove box with an oxygen content and a water content of no more than 0.01 ppm, dimethyl carbonate (DMC) and ethylene carbonate (EC) were mixed in a volume ratio of 1:1 to obtain a mixed solvent. The mixed solvent was then dehydrated using a 4Å molecular sieve, and lithium hexafluorophosphate (LiPF6) was added to a concentration of 1 mol / L. Dry ice was used to control the temperature during the stirring and dissolving process to ensure that the temperature fluctuation did not exceed 2°C. After the lithium hexafluorophosphate was completely dissolved, additive A was added to the lithium iron manganese phosphate battery electrolyte at a mass percentage of 1.5%, and the mixture was thoroughly stirred until the mixture was uniformly mixed to obtain a colorless and transparent lithium iron manganese phosphate battery electrolyte.

[0034] Comparative Example 4 A method for preparing a lithium manganese iron phosphate battery electrolyte comprises the following steps: In a high-purity argon glove box with an oxygen content and a water content of no more than 0.01 ppm, dimethyl carbonate (DMC) and ethylene carbonate (EC) were mixed in a volume ratio of 1:1 to obtain a mixed solvent. The mixed solvent was then dehydrated using a 4Å molecular sieve, and lithium hexafluorophosphate (LiPF6) was added to a concentration of 1 mol / L. Dry ice was used to control the temperature during the stirring and dissolving process to ensure that the temperature fluctuation did not exceed 2°C. After the lithium hexafluorophosphate was completely dissolved, 2.0% by mass of additive A was added to the lithium manganese iron phosphate battery electrolyte, and the mixture was thoroughly stirred until the mixture was uniformly mixed to obtain a colorless and transparent lithium manganese iron phosphate battery electrolyte.

[0035] Comparative Example 5 A method for preparing a lithium manganese iron phosphate battery electrolyte comprises the following steps: In a high-purity argon glove box with an oxygen content and a water content of no more than 0.01 ppm, dimethyl carbonate (DMC) and ethylene carbonate (EC) were mixed in a volume ratio of 1:1 to obtain a mixed solvent. The mixed solvent was then dehydrated using a 4Å molecular sieve, and lithium hexafluorophosphate (LiPF6) was added to a concentration of 1 mol / L. Dry ice was used to control the temperature during the stirring and dissolving process to ensure that the temperature fluctuation did not exceed 2°C. After the lithium hexafluorophosphate was completely dissolved, additive A was added to the lithium iron manganese phosphate battery electrolyte at a mass percentage of 2.5%, and the mixture was thoroughly stirred until the mixture was uniformly mixed to obtain a colorless and transparent lithium iron manganese phosphate battery electrolyte.

[0036] Comparative Example 6 A method for preparing a lithium manganese iron phosphate battery electrolyte comprises the following steps: In a high-purity argon glove box with an oxygen content and a water content of no more than 0.01 ppm, dimethyl carbonate (DMC) and ethylene carbonate (EC) were mixed in a volume ratio of 1:1 to obtain a mixed solvent. The mixed solvent was then dehydrated using a 4Å molecular sieve, and lithium hexafluorophosphate (LiPF6) was added to a concentration of 1 mol / L. Dry ice was used to control the temperature during the stirring and dissolving process to ensure that the temperature fluctuation did not exceed 2°C. After the lithium hexafluorophosphate was completely dissolved, 0.5% by mass of additive B was added to the lithium manganese iron phosphate battery electrolyte, and the mixture was thoroughly stirred until the mixture was uniformly mixed to obtain a colorless and transparent lithium manganese iron phosphate battery electrolyte.

[0037] Comparative Example 7 A method for preparing a lithium manganese iron phosphate battery electrolyte comprises the following steps: In a high-purity argon glove box with an oxygen content and a water content of no more than 0.01 ppm, dimethyl carbonate (DMC) and ethylene carbonate (EC) were mixed in a volume ratio of 1:1 to obtain a mixed solvent. The mixed solvent was then dehydrated using a 4Å molecular sieve, and lithium hexafluorophosphate (LiPF6) was added to a concentration of 1 mol / L. Dry ice was used to control the temperature during the stirring and dissolving process to ensure that the temperature fluctuation did not exceed 2°C. After the lithium hexafluorophosphate was completely dissolved, 1.0% by mass of additive B was added to the lithium manganese iron phosphate battery electrolyte, and the mixture was thoroughly stirred until the mixture was uniformly mixed to obtain a colorless and transparent lithium manganese iron phosphate battery electrolyte.

[0038] Comparative Example 8 The preparation method of the electrolyte comprises the following steps: In a high-purity argon glove box with an oxygen and water content of no more than 0.01 ppm, dimethyl carbonate (DMC) and ethylene carbonate (EC) were mixed in a volume ratio of 1:1 to obtain a mixed solvent. The mixed solvent was then dehydrated using a 4Å molecular sieve, and lithium hexafluorophosphate (LiPF6) was added to a concentration of 1 mol / L. Dry ice was used to control the temperature during the stirring and dissolving process to ensure that the temperature fluctuation did not exceed 2°C. The mixture was stirred thoroughly until it was mixed evenly, and finally a colorless and transparent electrolyte was obtained.

[0039] Comparative Example 9 The preparation method of the electrolyte comprises the following steps: In a high-purity argon glove box with an oxygen and water content of no more than 0.01 ppm, dimethyl carbonate (DMC) and ethylene carbonate (EC) were mixed in a volume ratio of 7:3 to obtain a mixed solvent. The mixed solvent was then dehydrated using a 4Å molecular sieve, and lithium hexafluorophosphate (LiPF6) was added to a concentration of 1 mol / L. Dry ice was used to control the temperature during the stirring and dissolving process to ensure that the temperature fluctuation did not exceed 2°C. The mixture was stirred thoroughly until it was mixed evenly, and finally a colorless and transparent electrolyte was obtained.

[0040] The following uses the electrolytes of Examples 1 to 4 and Comparative Examples 1 to 9 as examples to prepare lithium manganese iron phosphate batteries, and then conduct electrochemical performance tests: The preparation of the electrode includes: x Fe 1-xPO4 (0.5≤x≤0.7), conductive agent acetylene black and binder polyvinylidene fluoride (PVDF) are weighed in a mass ratio of 8:1:1, then mixed, dry-ground in an agate mortar for 10 minutes, and then N-methyl-2-pyrrolidone (NMP) is added as a solvent. The slurry is rapidly stirred with a magnetic bar for 12 hours to form a uniform slurry. The slurry is coated on aluminum foil, and after coating, it is first vacuum-dried at 120°C for 12 hours, and then rolled and stamped to finally obtain a circular lithium manganese iron phosphate battery positive electrode with a diameter of 15 mm; a metal lithium sheet is used as the negative electrode of the battery.

[0041] The separator is a polypropylene separator, which is a porous polymer film with a thickness of 20 μm; Assembly of button half-cells: In an argon atmosphere glove box (water content ≤ 0.01ppm, oxygen content ≤ 0.01ppm), the dried positive electrode sheet of the lithium iron phosphate battery, electrolyte, diaphragm and metal lithium sheet are placed in the operating area; during assembly, the negative electrode shell is turned upside down, the metal lithium sheet is grasped with tweezers and placed in the center of the negative electrode shell, and the electrolyte is evenly dripped on the surface of the lithium sheet using a rubber-tipped dropper, and then the diaphragm is covered, and the positive electrode sheet of the lithium iron phosphate battery is precisely stacked in the center of the diaphragm; after stacking the gasket and the spring in sequence, the positive and negative electrode shells are buckled together, lightly pressed and pre-tightened, and then transferred to the sealing machine, where it is kept for 5 seconds to complete the packaging, and finally a CR2025 button battery is formed.

[0042] Battery electrochemical performance test: Using a Xinwei battery testing system at 25±2°C, assembled button cells were subjected to constant current mode charge and discharge cycling tests at a 0.2C charge and discharge rate within a voltage range of 2.0V to 4.5V to evaluate the battery's long-cycle performance. The specific steps are as follows: The battery's first-cycle discharge capacity and the discharge capacity after 200 cycles were recorded. The capacity retention was calculated using the formula: 200-cycle capacity retention = (200-cycle discharge capacity / first-cycle discharge capacity) × 100%. The test data is shown in Table 1.

[0043] Metal manganese content test: After the above steps, the batteries corresponding to Example 2 and Comparative Example 8 were disassembled to obtain the negative electrode sheets of each; ICP test was performed to obtain the metal Fe and metal Mn contents. The results are as follows: Figure 2 shown.

[0044] Table 1 Electrochemical performance of examples and comparative examples As shown in Table 1 and Figure 1As shown, compared with no additives, the battery cycle performance and coulombic efficiency are improved after the addition of additive A. Similarly, compared with no additives, the cycle performance and coulombic efficiency of the battery are improved after the addition of additive B, and additives A and B. According to the cycle test results in Table 1, the capacity retention rate of Comparative Example 3 at 25°C is higher than that of Comparative Example 1, Comparative Example 2, Comparative Example 4 and Comparative Example 5, indicating that when the amount of additive A added is insufficient or excessive, its effect on improving the cycle performance is not good. Under 25°C conditions, the battery capacity retention rate of Comparative Example 8 and Comparative Example 9 without Additive A is significantly reduced compared with Comparative Example 1, confirming that Additive A can effectively improve the cycle performance.

[0045] Comparing the experimental data from Comparative Examples 6 and 7 shows that as the amount of Additive B increases, the battery system's cycle capacity retention and average Coulombic efficiency decrease. This indicates that excessive addition of Additive B significantly increases charge transfer resistance at the electrode / electrolyte interface, leading to increased kinetic polarization, which in turn reduces lithium-ion intercalation and deintercalation efficiency and increases side reactions. Compared to the test data from the comparative examples, the capacity retention of Examples 1 through 4 shows an upward trend, demonstrating that the dual-additive system improves cycle performance more effectively than using either additive alone, with a more significant synergistic effect.

[0046] As described in the technical analysis of the previous section of the present invention, the dissolution of manganese elements causes multi-dimensional damage to lithium-ion batteries. On the one hand, the lattice structure of the positive electrode active material collapses, causing capacity decay. On the other hand, the dissolved manganese ions produce by-products by accelerating the oxidation and decomposition of the electrolyte, exacerbating the irreversible consumption of active lithium. Based on this, it is necessary to construct a multi-level protection system of electrode-electrolyte-interface to effectively inhibit the chain failure behavior of positive electrode structure degradation, continuous decomposition of electrolyte and interface instability caused by manganese ion migration, so as to achieve a synergistic improvement in the comprehensive performance of the lithium-ion battery system. Figure 2 As shown, compared with comparative example 8 without additives, the battery system of the present invention can significantly inhibit the precipitation of iron and manganese elements in the lithium iron manganese phosphate positive electrode. After analysis, additive A can build a strong CEI layer on the surface of lithium iron manganese phosphate because of its high HOMO energy, improve the interface stability of CEI, insulate lithium iron manganese phosphate from the electrolyte, reduce side reactions, and thus improve electrochemical performance. Additive B can absorb HF acid in the electrolyte and participate in the construction of the CEI film. The synergistic effect of the two additives produces a low Li + CEI films with high electrical resistance and high voltage stability.

[0047] Based on a systematic study of the combination ratios of various functional additives within specific preferred concentration ranges, and confirmed through electrochemical performance testing, the composite additive system corresponding to Example 2 exhibits the best overall performance. The remaining combinations are not listed in the Examples. This electrolyte system, through the synergistic action of multiple components, exhibits excellent cycling stability and high coulombic efficiency while suppressing manganese ion dissolution. This technical solution provides an innovative solution to the problems of increased polarization and capacity fade in lithium manganese iron phosphate batteries.

[0048] In this specification, the terms such as "embodiment" and "example" are intended to indicate that the relevant features, structures or methods can be reasonably combined or adjusted to be applied in at least one embodiment of the present invention, and such descriptions do not constitute a limitation on the scope of protection. Through experimental verification, the electrolyte additive of the present invention can significantly improve the rate performance and life of lithium manganese iron phosphate batteries, reduce battery impedance and effectively inhibit the precipitation of manganese elements. It should be clear that the above embodiments are only exemplary descriptions of the technical solutions. Any modification, equivalent substitution or local adjustment based on the core concept of the present invention is a legitimate extension of the present invention as long as it does not deviate from the technical essence and protection scope defined by the claims. In addition, reasonable changes or adaptive improvements to specific implementation details by those skilled in the art, under the premise of following the technical principles of the present invention, should also be regarded as falling within the scope of protection of the present invention.

Claims

1. A lithium manganese iron phosphate battery electrolyte additive, characterized in that: The electrolyte additive for lithium manganese iron phosphate battery consists of additive A and additive B; Additive A is a borate compound, and its structural formula is , wherein R1, R2, R3, and R4 are independently selected from one of pentafluorophenyl, oxalate, and F; Additive B is selected from at least one of trimethylsiloxane phosphate, trimethylsiloxane carbonate, trimethylsilyl chloride, trimethylsilyl isocyanate, trimethylsilyl borate, trimethylsiloxane, hexamethyldisiloxane, trimethylsilyl glycol dimethyl ether, trimethyltrifluoropropylsilane, trimethylsilyl perfluoropolyether, and trimethylsilyl cyanide; The mass ratio of additive A to additive B is 1~5:

1.

2. The electrolyte additive for lithium manganese iron phosphate batteries according to claim 1, characterized in that: The mass ratio of additive A to additive B is 1~3:

1.

3. A lithium manganese iron phosphate battery electrolyte, characterized in that: The electrolyte additive for lithium manganese iron phosphate batteries according to any one of claims 1 to 2, a lithium salt and an organic solvent.

4. The lithium manganese iron phosphate battery electrolyte according to claim 3, characterized in that The mass percentage of additive A in the lithium manganese iron phosphate battery electrolyte is 0.5wt%~2.5wt%.

5. The lithium manganese iron phosphate battery electrolyte according to claim 4, characterized in that: The mass percentage of additive A in the lithium manganese iron phosphate battery electrolyte is 1.5 wt %.

6. The lithium manganese iron phosphate battery electrolyte according to claim 3, characterized in that The mass percentage of additive B in the lithium manganese iron phosphate battery electrolyte is 0.5wt%~1.5wt%.

7. The lithium manganese iron phosphate battery electrolyte according to claim 3, characterized in that The organic solvent is a carbonate solvent, and the mass percentage of the carbonate solvent in the lithium manganese iron phosphate battery electrolyte is 65wt%~85wt%.

8. The lithium manganese iron phosphate battery electrolyte according to claim 3, characterized in that The mass percentage of lithium salt in the lithium manganese iron phosphate battery electrolyte is 10wt%~20wt%.

9. A method for preparing the lithium manganese iron phosphate battery electrolyte according to claim 3, characterized in that: The steps include: An organic solvent, a lithium salt and an electrolyte additive for a lithium iron manganese phosphate battery are mixed and stirred evenly to prepare an electrolyte for a lithium iron manganese phosphate battery.

10. Use of the lithium manganese iron phosphate battery electrolyte according to claim 3 in the preparation of a lithium manganese iron phosphate battery.

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