Electrolyte and Lithium-ion Battery

Abstract

This invention provides an electrolyte and a lithium-ion battery. The electrolyte includes a first additive and a second additive. The first additive has a molecular structure as shown in Formula I: Formula I; the second additive has a molecular structure as shown in Formula II: Formula II. The electrolyte of this application embodiment, by introducing the first additive and the second additive, helps to improve the cycle performance and fast charging performance of the battery under low electrolyte volume conditions.

Description

Technical Field

[0001] This invention relates to the field of battery technology, specifically to an electrolyte and a lithium-ion battery. Background Technology

[0002] With the rapid popularization of portable electronic devices, new energy electric vehicles and energy storage systems, the market has put forward higher requirements for the performance of secondary batteries: higher energy density, excellent fast charging performance, longer cycle life and higher safety.

[0003] In related technologies, to meet the performance requirements of secondary batteries, the energy density can be increased by raising the charging voltage or increasing the capacity of the active materials. However, increasing the specific capacity of the active materials may lead to higher coating density and greater compaction density, further compressing the internal voids of the cell and resulting in a lower electrolyte injection volume. Traditional electrolytes have lower liquid retention capacity when the injection volume is low, and low liquid retention capacity degrades the battery's fast charging and cycle performance, often resulting in a sudden drop in electrolyte level during the later stages of cycling. The requirements of fast charging further exacerbate the risk of this drop in electrolyte level. Summary of the Invention

[0004] The embodiments of the present invention provide an electrolyte and a lithium-ion battery, which can improve the technical problem that the electrolyte cannot maintain the battery with excellent cycle performance and fast charging performance when the electrolyte volume is low.

[0005] In a first aspect, embodiments of the present invention provide an electrolyte, the electrolyte comprising:

[0006] First additive and second additive, wherein the first additive has a molecular structure as shown in Formula I:

[0007] Formula I;

[0008] The second additive has a molecular structure as shown in Formula II:

[0009] Formula II;

[0010] in,

[0011] R1, R2, and R3 are each independently selected from halogen atoms, C1-C 10 Chain alkyl, C3-C 10 Any one of the cycloalkyl groups;

[0012] R4 is independently selected from any one of C2-C5 alkenyl or C2-C5 alkynyl groups;

[0013] The dashed lines represent conjugate π bonds.

[0014] Optionally, in one embodiment, the second additive is selected from at least one of the following compounds:

[0015] Formula II-1;

[0016] Formula II-2;

[0017] Formula II-3;

[0018] Formula II-4.

[0019] Optionally, in one embodiment, the mass ratio of the first additive to the second additive is 1:(1~4).

[0020] Optionally, in one embodiment, the total mass of the electrolyte is used as a reference.

[0021] The first additive has a mass percentage content greater than or equal to 0.1% and less than or equal to 4%; and / or

[0022] The second additive has a mass percentage content greater than or equal to 0.1% and less than or equal to 4%.

[0023] Optionally, in one embodiment, the mass percentage of the first additive is greater than or equal to 0.2% and less than or equal to 2%; and / or

[0024] The second additive has a mass percentage content greater than or equal to 0.2% and less than or equal to 2%.

[0025] Optionally, in one embodiment, the electrolyte injection coefficient is 2.9 g / Ah to 3.2 g / Ah.

[0026] Optionally, in one embodiment, the electrolyte further includes a lithium salt, which includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium difluorooxalate borate, lithium bis(trifluoromethylsulfonyl)imide, and lithium bis(oxalate borate).

[0027] Optionally, in one embodiment, the lithium salt has a mass percentage content of 10% to 18%, based on the total mass of the electrolyte.

[0028] Optionally, in one embodiment, the electrolyte further includes a non-aqueous solvent, which includes at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl formate, ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, and ethylene glycol dimethyl ether.

[0029] Secondly, embodiments of the present invention provide a lithium-ion battery, the lithium-ion battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte as described above. The positive electrode comprises a positive electrode active material; the positive electrode active material comprises LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, Li... 1+a Mn 1-x M x O2, LiCo 1-x M x O2, LiFe 1-x M x PO4 and Li2Mn 1-x M x At least one of O4; wherein M is selected from at least one of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V and Ti, 0≤a<0.2, 0≤x<1.

[0030] The beneficial effects of the embodiments of the present invention are as follows:

[0031] In embodiments of the present invention, the electrolyte includes a first additive and a second additive. The second additive preferentially decomposes before the solvent in the electrolyte undergoes oxidative decomposition, forming a stable positive electrode-electrolyte interface film. The second additive is an ionic liquid additive containing multiple functional groups. Its introduction can significantly increase the ion transport number of the electrolyte, resulting in a higher conductivity. This helps to improve the ion transport rate of the electrolyte and enhance the fast-charging performance of the battery cell. However, under low electrolyte retention conditions, the electrolyte is not fully filled in the electrode pores and the separator, leading to a longer molecular diffusion path for the second additive in the electrolyte. Furthermore, due to the large molecular weight of the second additive, it cannot diffuse into the entire electrode interior in a timely and uniform manner, resulting in a high additive concentration on the electrode surface and uneven interfacial reaction. Especially during high-rate charge and discharge, the interfacial reaction rate is fast, and the local additive is rapidly consumed, while new additive cannot be replenished in time, causing the interfacial stability of that region to deteriorate rapidly. The first additive has strong diffusion properties and easily decomposes into highly electronegative sulfate groups when forming films at the positive or negative electrode. These sulfate groups can attack the cage-like anionic groups in the second additive, causing their phosphorus-oxygen bonds to break. The interaction between the two forms a uniform and stable small molecular structure. This small molecular structure can diffuse evenly throughout the entire electrode, solving the problem of uneven distribution caused by limited electrolyte transport. Moreover, the decomposition products of the first and second additives include inorganic salt components such as lithium alkyl sulfonate, lithium phosphate, and LiF. These inorganic salt components can improve the stability of the electrolyte-electrode interface under low electrolyte retention coefficient. Detailed Implementation

[0032] The technical solution of the present invention will be clearly and completely described below with reference to embodiments thereof. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Furthermore, it should be understood that the specific embodiments described herein are only for illustration and explanation of the present invention and are not intended to limit the present invention.

[0033] In related technologies, to meet the performance requirements of secondary batteries, the energy density can be increased by raising the charging voltage or increasing the capacity of the active materials. However, increasing the specific capacity of the active materials may lead to higher coating density and greater compaction density, further compressing the internal voids of the cell and resulting in a lower electrolyte injection volume. Traditional electrolytes also have lower liquid retention capacity when the injection volume is low. Under low electrolyte retention conditions, the cycle performance of the battery may decrease, especially in the later stages of the cycle, where a sudden drop in electrolyte level often occurs. Moreover, during high-rate charge and discharge, the decomposition and consumption of the electrolyte are accelerated, exacerbating interfacial side reactions in the cell and leading to poor interfacial stability in local electrode areas, thus affecting the battery's fast-charging performance.

[0034] In view of this, embodiments of this application provide an electrolyte and a lithium-ion battery, aiming to improve the problem that the electrolyte cannot maintain excellent cycle performance and fast charging performance under low electrolyte volume conditions.

[0035] According to a first aspect of the embodiments of this application, an electrolyte is provided, the electrolyte comprising:

[0036] First additive and second additive.

[0037] The first additive has a molecular structure as shown in Formula I:

[0038] Formula I;

[0039] The second additive has a molecular structure as shown in Formula II:

[0040] Formula II;

[0041] in,

[0042] R1, R2, and R3 are each independently selected from halogen atoms, C1-C 10 Chain alkyl, C3-C 10 Any one of the cycloalkyl groups;

[0043] R4 is independently selected from any one of C2-C5 alkenyl or C2-C5 alkynyl groups;

[0044] The dashed lines represent conjugate π bonds.

[0045] The electrolyte in this embodiment includes a first additive and a second additive. The second additive preferentially decomposes before the solvent in the electrolyte oxidizes and decomposes, forming a stable positive electrode-electrolyte interface film. The second additive is an ionic liquid additive containing multiple functional groups. Its introduction can significantly increase the ion transport number of the electrolyte, resulting in higher conductivity. This helps to improve the ion transport rate of the electrolyte and enhance the fast-charging performance of the battery cell. However, under low electrolyte retention conditions, the electrolyte is not fully filled in the electrode pores and the separator, which leads to a longer molecular diffusion path for the second additive in the electrolyte. Furthermore, due to the large molecular weight of the second additive, it cannot diffuse into the entire electrode interior in a timely and uniform manner, resulting in a high additive concentration on the electrode surface and uneven interfacial reaction. Especially during high-rate charge and discharge, the interfacial reaction rate is fast, and the local additive is rapidly consumed, while new additive cannot be replenished in time, causing the interfacial stability in that area to deteriorate rapidly.

[0046] In this embodiment, the first additive is introduced into the second additive. Due to its strong diffusivity, it easily decomposes into a sulfate group with strong electronegativity when forming a film at the positive or negative electrode. This sulfate group can attack the cage-like anionic group in the second additive, causing its phosphorus-oxygen bond to break. The two interact to form a uniform and stable small molecule structure. This small molecule structure can diffuse evenly throughout the entire electrode, solving the problem of uneven distribution caused by limited electrolyte transport. Moreover, the decomposition products of the first and second additives include inorganic salt components such as lithium alkyl sulfonate, lithium phosphate, and LiF. The inorganic salt components can improve the stability of the electrolyte-electrode interface under low electrolyte retention coefficient.

[0047] It should also be noted that the sulfate groups in the first additive molecule are easily hydrolyzed, especially in the presence of trace amounts of moisture, decomposing to produce acidic sulfuric acid substances (such as H2SO4). Commercially available electrolyte lithium salts (such as LiPF6) are easily hydrolyzed to generate HF (hydrofluoric acid) and PF5. The ring-opening decomposition products of the first additive during storage may catalyze this process, further increasing the electrolyte acidity. The embodiments of this application introduce a second additive, whose cage-like conjugated ring structure facilitates further reaction with the sulfate groups of the first additive, resulting in ring-opening polymerization. This avoids the acidity increase problem caused by adding the first additive during storage. The two interact to improve the thermal and chemical stability of the electrolyte, forming a highly conductive and stable electrolyte.

[0048] It should also be noted that the electrolyte of this application embodiment contains a compound of Formula I as a first additive and a compound of Formula II as a second additive. The compound of Formula I undergoes ring-opening at the low-potential negative electrode, which helps to copolymerize with the compound of Formula II at the negative electrode to form a film, thereby forming a dense protective film on the surface of the negative electrode, effectively inhibiting the decomposition of solvent and lithium salt in the electrolyte. Moreover, the compound of Formula II can preferentially oxidize and decompose before the solvent, forming a stable positive electrode-electrolyte interface film. The two can form the framework of a polymer bilayer interface film, improving the stability of the interface film. The framework of this bilayer interface film can avoid continuous contact between the electrode material and the electrolyte, preventing electrolyte decomposition while reducing damage to the electrode material structure.

[0049] It should be further explained that when the electrolyte uses compounds of Formula I and Formula II, the two compounds together form a bilayer interfacial film with high conductivity and high thermal stability at the positive electrode interface, which effectively reduces the interfacial side reactions of the battery electrolyte, keeping the electrolyte in a low-consumption state; it helps to improve the interfacial stability between the electrolyte and the electrode, ensures a high-conductivity and stable electrolyte system, and further improves the fast-charging performance of the battery.

[0050] Therefore, the electrolyte in this application embodiment enables the battery to have excellent cycle performance and fast charging performance with a low liquid retention coefficient.

[0051] In some embodiments of this application, the second additive is selected from at least one of the following compounds:

[0052] Formula II-1;

[0053] Formula II-2;

[0054] Formula II-3;

[0055] Formula II-4.

[0056] By adopting the above scheme, compared with Formula II-1, Formula II-2 has a denser electron cloud of alkyne group, resulting in a stronger interaction with lithium ions, which may affect the ion migration rate. Therefore, Formula II-2 has lower reactivity than Formula II-1. In addition, the alkyne group has greater rigidity, which can further improve the film density and affect the fast charging performance of the battery.

[0057] Compared with Formula II-1, R1, R2 and R3 of the amino cation moiety are selected from C5 cycloalkyl groups. C5 cycloalkyl groups have certain steric hindrance, which is beneficial to improve the structural stability of the amino cation and enhance the thermal stability of the electrolyte. Moreover, the selection of R1, R2 and R3 from C5 cycloalkyl groups can also change the electron cloud distribution around the amino cation. The denser the electron cloud distribution, the more it affects the conductivity of lithium ions, thereby reducing the cycle performance and fast charging performance of the battery.

[0058] It should be noted that the CAS number of Formula II-1 is 2681338-34-9.

[0059] In some embodiments of this application, the mass ratio of the first additive to the second additive is 1:(1~4). Exemplarily, the mass ratio of the first additive to the second additive can be 1:1, 1:2, 1:3, 1:4, or any mass ratio within the range of two adjacent mass ratios mentioned above.

[0060] By adopting the above scheme, the first additive and the second additive are compounded in a suitable mass ratio, which helps to further improve the battery's excellent cycle performance and fast charging performance under low liquid retention coefficient.

[0061] In some embodiments of this application, based on the total mass of the electrolyte, the mass percentage of the first additive is greater than or equal to 0.1% and less than or equal to 4%; preferably, the mass percentage of the first additive is greater than or equal to 0.1% and less than or equal to 2%. Exemplarily, the mass percentage of the first additive can be 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, or any value between two adjacent values ​​mentioned above.

[0062] By adopting the above scheme, the first additive with an appropriate mass percentage provides a suitable amount of sulfate groups, which helps to attack the cage-like anionic groups of the second additive, thereby forming a stable small molecular structure. This small molecular structure helps to diffuse evenly throughout the entire electrode, which helps to solve the problem of uneven distribution of the second additive in the electrode leading to uneven interface stability. At the same time, it can also solve the problem of the second additive having a longer molecular diffusion path.

[0063] In some embodiments of this application, based on the total mass of the electrolyte, the mass percentage of the second additive is greater than or equal to 0.1% and less than or equal to 4%; further, the mass percentage of the second additive is greater than or equal to 0.1% and less than or equal to 2%. Exemplarily, the mass percentage of the second additive is 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, and any value between two adjacent values ​​mentioned above.

[0064] By adopting the above scheme, the second additive with an appropriate mass percentage helps to reduce the acidity increase caused by the introduction of the first additive. Moreover, the second additive is preferentially oxidized and decomposed before the solvent, which can form a stable positive electrode-electrolyte interface film. At the same time, the second additive can also copolymerize with the first additive to form a film, together forming the framework of the double-layer interface film, thereby improving the stability of the interface film, avoiding continuous contact between the electrode material and the electrode liquid, effectively preventing electrolyte decomposition while reducing the damage to the electrode material structure.

[0065] In some embodiments of this application, the electrolyte may further include a lithium salt, which may include at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium difluorooxalate borate, lithium bis(trifluoromethanesulfonyl)imide, and lithium bis(oxalate borate).

[0066] In some embodiments of this application, the mass percentage of lithium salt is 10% to 18% based on the total mass of the electrolyte. Exemplarily, the mass percentage of lithium salt is 10%, 10.5%, 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14%, 14.5%, 15%, 15.5%, 16%, 16.5%, 17%, 17.5%, 18%, and any value between two adjacent values.

[0067] By adopting the above scheme, lithium salts meet the aforementioned conditions, which helps ions in the electrolyte to migrate and transport efficiently and stably, thereby improving the rate performance and cycle performance of the battery. Furthermore, it helps maintain the chemical stability of the electrolyte, reduces side reactions during charge and discharge, and contributes to extending the battery's cycle life and improving its safety.

[0068] In some embodiments of this application, the electrolyte may further include a non-aqueous solvent, which may include at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl formate, ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, ether, ethylene glycol dimethyl ether, and so on.

[0069] It should be noted by way of example that the non-aqueous solvent may include at least one of ethyl acetate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate.

[0070] In some embodiments of this application, the mass percentage of the non-aqueous solvent is 74% to 90%, based on the total mass of the electrolyte. Exemplarily, the mass percentage of the non-aqueous solvent is 74%, 78%, 82%, 86%, 88%, 89%, 90%, and any value between two adjacent values.

[0071] By employing the above-described scheme, the non-aqueous solvent meets the aforementioned conditions, enabling lithium ions to migrate efficiently and distribute uniformly in the electrolyte, thereby improving the battery's charge-discharge efficiency and cycle stability. Furthermore, it facilitates the formation of a stable SEI film, reducing electrolyte decomposition. Simultaneously, it optimizes the battery's thermal stability, reducing side reactions under high-temperature conditions and contributing to enhanced battery performance.

[0072] According to a second aspect of the embodiments of this application, a lithium-ion battery is provided, the lithium-ion battery including a positive electrode, a negative electrode, a separator, and an electrolyte as described above.

[0073] By adopting the above-described solution, the lithium-ion battery possesses all the characteristics and advantages of the aforementioned electrolyte, which will not be repeated here. In summary, it has at least the advantage of maintaining excellent cycle performance and fast-charging performance with low electrolyte levels.

[0074] In some embodiments of this application, the positive electrode includes a positive electrode active material selected from LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, and Li 1+a Mn 1-x M x O2, LiCo 1-x M x O2, LiFe 1-x M x PO4 and Li2Mn 1- x M x At least one of O4; M is selected from at least one of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V, Ti, B and F, 0≤a<0.2, 0≤x<1.

[0075] For example, 0≤a≤0.19, 0.05≤a≤0.15, 0.08≤a≤0.13, 0.1≤a≤0.12; 0≤x≤0.9, 0.1≤x≤0.8, 0.2≤x≤0.7, 0.3≤x≤0.6, 0.4≤x≤0.5, etc.

[0076] Understandably, Li 1+a Mn 1-x M x O2, LiCo 1-x M x O2, LiFe 1-x M x PO4 and Li2Mn 1-x M xThe choice of M in each chemical formula of O4 is independent and does not affect each other; they can be the same or different. Similarly, in the list of positive electrode active materials above, the choices of a and x are also independent and do not affect each other; they can be the same or different.

[0077] The present application will be specifically described below through specific embodiments. These embodiments are only some embodiments of the present application and are not intended to limit the present application. Unless otherwise specified, the raw materials used in the following embodiments are all commercially available products.

[0078] Example 1

[0079] 1. Preparation of positive electrode sheet

[0080] Lithium iron phosphate, a positive electrode active material, carbon black and carbon nanotubes, and polyvinylidene fluoride (PVDF), a binder, were added to N-methylpyrrolidone (NMP) in a mass ratio of 94.5:4:1.5 to prepare a positive electrode slurry. The positive electrode slurry was coated on the upper and lower surfaces of an aluminum foil, dried, and then cold-pressed. After trimming, cutting, and slitting, a positive electrode sheet was prepared, with a single-sided positive electrode material layer thickness of 72 μm.

[0081] 2. Preparation of negative electrode sheet

[0082] A negative electrode slurry was prepared by mixing graphite with conductive carbon black, binder styrene-butadiene rubber (SBR), and thickener (CMC) in deionized water at a mass ratio of 94.5:2:2:1.5. The negative electrode slurry was coated on the upper and lower surfaces of copper foil and dried. Then, it was cold-pressed, trimmed, cut into sheets, and slit to form a negative electrode sheet with a single-sided negative electrode material layer thickness of 110 μm.

[0083] 3. Preparation of electrolyte

[0084] In an argon-filled glove box (moisture < 10 ppm, oxygen < 1 ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and ethyl acetate (EA) were mixed uniformly in a mass ratio of 3:3:4. Sufficiently dried lithium hexafluorophosphate (LiPF6) (10%) and lithium bis(fluorosulfonyl)imide (LiFSI) (4.6%) were added to the mixed solvent and mixed. After mixing, lithium salt, first additive, second additive, and other additives were added according to the mass percentage of each component, and the mixture was mixed again to obtain the electrolyte. The other additives used were ethylene carbonate, with a mass percentage of 3% in the electrolyte.

[0085] 4. Diaphragm

[0086] A 10μm polyethylene film was used as the diaphragm.

[0087] 5. Battery manufacturing

[0088] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to isolate them. The cells are then wound to obtain a bare cell. The tabs are welded on, and the bare cell is placed in an outer package. The electrolyte prepared above is injected into the dried cell. The cells are then encapsulated, left to stand, formed, and shaped to complete the battery manufacturing process.

[0089] It is understood that in the electrolyte, when the total amount of additives (the embodiments of this application include the first additive, the second additive and other additives) changes between different embodiments, the content of solvent also changes accordingly. For example, if the total amount of additives increases by 1%, the content of solvent will decrease by 1%, but the ratio of EC, EMC and EA in the solvent is still 3:3:4.

[0090] The preparation methods of the batteries in Examples 2-25 and Comparative Examples 1-5 are the same as those in Example 1, except that the composition of the additives in the electrolyte is different, as shown in Table 1.

[0091] Table 1

[0092]

[0093] Performance testing:

[0094] (1) Room temperature cycling performance:

[0095] At 25°C, charge to 3.65V using 2C constant current and constant voltage, let stand for 5 minutes, then discharge to 2.5V using 1C constant current, and calculate the capacity retention rate after 1500 cycles.

[0096] Capacity retention rate (%) = (Discharge capacity at 1500th discharge / Discharge capacity at 1st discharge) × 100%;

[0097] (2) Liquid retention efficiency:

[0098] After the secondary battery underwent the above-mentioned room temperature cycling performance test, an electrolyte retention test was performed. After opening the secondary battery, 3g of dichloromethane was injected into the electrolyte inlet. The secondary battery was then placed in an ultrasonic machine and sonicated for 60 minutes to ensure uniform mixing of the dichloromethane and electrolyte. After the battery cell was left to stand at room temperature in a sealed container for one day, the battery was disassembled in a glove box with a moisture content below 0.1ppm and an oxygen content below 0.1ppm. The electrolyte was removed, and the mass percentage of dichloromethane in the electrolyte was determined using gas chromatography-mass spectrometry (GC-MS) / ion chromatography. The mass of the remaining electrolyte was calculated using the injected dichloromethane volume and the mass percentage of dichloromethane in the electrolyte.

[0099] The mass of the remaining electrolyte is: ;

[0100] The electrolyte retention efficiency (%) after 1500 cycles at room temperature (2C / 1C) is: The electrolyte injection volume refers to the amount of electrolyte injected during the secondary battery manufacturing process.

[0101] (3) Fast charging performance:

[0102] The battery was placed in a 25℃ environment and charged at a constant current of 1C to the rated voltage. Then, it was charged at a constant voltage until the cutoff current reached 0.05C, left to stand for 5 minutes, and then discharged at a constant current of 1C to 2.5V. The charging capacity was recorded as Ccharge. After charging at a constant rate to the rated voltage, it was charged at a constant voltage until the current decreased to 0.05C, left to stand for 5 minutes, and then discharged at a constant current of 1C to 2.5V, left to stand for 5 minutes. This constitutes one charge-discharge cycle. This process was repeated 3 times. The battery was then tested at 0.5C, 1C, 2C, 3C, and 4C rates. The charging capacity of the constant current segment in the last cycle at 4C was recorded as C4. The 4C constant current charge ratio was calculated as (C4 / Ccharge) × 100%.

[0103] (4) High-temperature storage performance:

[0104] At 60℃, the lithium-ion battery was charged to 3.65V using a 1C constant current and constant voltage method. The initial thickness of the lithium-ion battery was measured at this point. Then, the battery was stored at 60℃ for 30 days to test its thickness.

[0105] Wherein, the expansion rate (%) = (thickness after storage - initial thickness) / initial thickness × 100%;

[0106] After high-temperature storage, the lithium-ion battery is discharged to 2.5 V at 1C, and the capacity retention rate of the battery is measured and calculated. The calculation formula is as follows: Capacity retention rate (%) = Retained capacity / Initial capacity × 100%.

[0107] Please refer to Table 2 for the test results.

[0108] Table 2

[0109]

[0110] Compared to Examples 1-8, the difference lies only in the content of the second additive. As shown in Tables 1-2, a suitable mass percentage of the second additive helps reduce the acidity increase caused by the introduction of the first additive. Furthermore, the second additive preferentially oxidizes and decomposes before the solvent, forming a stable positive electrode-electrolyte interface film. Simultaneously, the second additive can copolymerize with the first additive to form a bilayer interface film framework, thereby improving the stability of the interface film, preventing continuous contact between the electrode material and the electrode solution, effectively preventing electrolyte decomposition while reducing damage to the electrode material structure. However, the content of the second additive should not be too high, as this will further degrade the uniformity of the second additive's dispersion within the electrode under low electrolyte retention conditions. Therefore, a mass percentage of 0.5% to 2% for the second additive is preferable.

[0111] Compared with Examples 1-5, Comparative Examples 1-2 did not add the second additive, Comparative Examples 3-4 did not add the first additive, and Comparative Example 5 did not add either the first or second additive. However, Example 2 used a combination of the first and second additives. As shown in Tables 1-2, Example 2 significantly improved its cycle performance, fast charging performance, and high-temperature storage performance. This is because the second additive preferentially decomposes before the electrolyte solvent oxidizes and decomposes, forming a stable positive electrode-electrolyte interface film. Simultaneously, the second additive is an ionic liquid additive containing multiple functional groups. Its introduction significantly increases the ion transference number in the electrolyte, resulting in higher conductivity, which helps improve the ion transport rate of the electrolyte and enhance the fast charging performance of the battery cell. Furthermore, the amino cationic groups of the second additive stabilize the lithium salt anions, ensuring that the lithium salt anions are stable and not easily decomposed during cycle storage. Introducing the first additive into the second additive is beneficial because the first additive has strong diffusivity and readily decomposes into highly electronegative sulfate groups during film formation at the positive or negative electrode. These sulfate groups can attack the cage-like anionic groups in the second additive, causing the phosphorus-oxygen bonds to break. The interaction between the two forms a uniform and stable small molecular structure, which can diffuse evenly throughout the entire electrode, solving the problem of uneven distribution caused by limited electrolyte transport. Furthermore, the decomposition products of the first and second additives include inorganic salt components such as lithium alkyl sulfonate, lithium phosphate, and LiF. These inorganic salt components can improve the stability of the electrolyte-electrode interface under low electrolyte retention coefficients. Therefore, Example 2 uses a combination of the first and second additives, which contributes to excellent cycle performance and fast-charging performance under low electrolyte retention coefficients.

[0112] Compared with Examples 2 and 9-15, the difference lies only in changing the content of the first additive. Compared with Table 1-2, as the content of the first additive increases, it helps to provide an appropriate amount of sulfate ester groups, which is more conducive to attacking the cage-like anionic groups of the second additive. The interaction between the two is conducive to forming a stable small molecular structure. This small molecular structure can diffuse more evenly into the entire electrode, which is more conducive to improving the interface stability of the electrode.

[0113] Based on Examples 16-18 and 10-12 and Tables 1-2, it can be seen that, compared to Examples 10-12, increasing the content of the second additive in Examples 16-18 to 1% results in excellent cycle performance and fast charging performance, as well as superior storage stability at high temperatures. Therefore, it can be concluded that a mass ratio of the first additive to the second additive within the range of 0.12% to 9% can guarantee the cycle and fast charging performance of the battery, and is not significantly related to the total amount of additives.

[0114] Based on Examples 2 and 19-21 and Table 1-2, it can be seen that the second additive in Example 19 contains an alkyne group. The electron cloud of the alkyne group is more concentrated, resulting in a stronger interaction with lithium ions, which affects the ion migration rate and the battery's cycle performance. Furthermore, the alkyne group has greater rigidity, which can further improve the film density, but also affects the battery's fast-charging performance. In the second additive of Example 20, R1, R2, and R3 of the aminocation moiety are selected from C5 cycloalkyl groups. C5 cycloalkyl groups have certain steric hindrance, which is beneficial for improving the structural stability of the aminocation and enhancing the thermal stability of the electrolyte; however, C5 cycloalkyl groups may increase the density of the electron cloud distribution, affecting the conductivity of lithium ions and thus reducing the battery's cycle performance.

[0115] Compared with Examples 2 and 22-23, Examples 22-23 replaced the positive electrode active material of Example 2. As can be seen from Tables 1-2, under the same liquid injection conditions, the battery capacity retention rate and fast charging performance are not much different from those of Example 2, but the cycle performance is slightly affected.

[0116] Compared with Examples 2 and 24-25, Examples 24-25 further reduced the amount of liquid injected. As shown in Tables 1-2, the liquid retention efficiency, capacity retention rate, fast charging performance and cycle performance of the battery will all be affected.

[0117] The embodiments of the present invention have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. An electrolyte, characterized in that, The electrolyte injection coefficient is from 2.9 g / Ah to 3.2 g / Ah; The electrolyte includes a first additive and a second additive; The first additive has a molecular structure as shown in Formula I: Formula I; The second additive is selected from at least one of the following compounds: Formula II-1; Formula II-2; Formula II-3; Formula II-4; The mass ratio of the first additive to the second additive is 1:(1~4).

2. The electrolyte according to claim 1, characterized in that, Using the total mass of the electrolyte as a reference, The first additive has a mass percentage content greater than or equal to 0.1% and less than or equal to 4%; and / or The second additive has a mass percentage content greater than or equal to 0.1% and less than or equal to 4%.

3. The electrolyte according to claim 2, characterized in that, The first additive has a mass percentage content greater than or equal to 0.2% and less than or equal to 2%; and / or The second additive has a mass percentage content greater than or equal to 0.2% and less than or equal to 2%.

4. The electrolyte according to claim 2, characterized in that, The electrolyte further includes lithium salts, which include at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium difluorooxalate borate, lithium bis(trifluoromethylsulfonyl)imide, and lithium bis(oxalate borate).

5. The electrolyte according to claim 4, characterized in that, Based on the total mass of the electrolyte, the lithium salt has a mass percentage content of 10% to 18%.

6. The electrolyte according to claim 1, characterized in that, The electrolyte further includes a non-aqueous solvent, which includes at least one of ethylene carbonate, propylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl formate, ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, methyl butyrate, ethyl butyrate, and ethylene glycol dimethyl ether.

7. A lithium-ion battery, characterized in that, The lithium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte as described in any one of claims 1 to 6; The positive electrode sheet includes a positive electrode active material, which includes LiCoO2, LiMn2O4, LiMnO2, Li2MnO4, LiFePO4, and Li 1+a Mn 1-x M x O2, LiCo 1-x M x O2, LiFe 1-x M x PO4 and Li2Mn 1-x M x At least one of O4; wherein M is selected from at least one of Ni, Co, Mn, Al, Cr, Mg, Zr, Mo, V and Ti, 0≤a<0.2, 0≤x<1.

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