Lithium ion battery electrolyte and lithium ion battery

Abstract

This invention discloses a lithium-ion battery electrolyte and a lithium-ion battery containing the electrolyte. The electrolyte comprises a main lithium salt, an organic solvent, and a first additive, wherein the first additive is a triazine compound with the structure shown in formula (A); and a second additive, wherein the second additive is lithium difluorophosphate-based fluoroborate or lithium difluorophosphate-based fluorophosphate with the structure shown in formula (B). The definitions of each substituent in formulas (A) and (B) are detailed in the specification. The first additive accounts for 0.01 to 5.0 wt% of the total mass of the electrolyte; the second additive accounts for 0.01 to 10.0 wt% of the total mass of the electrolyte. The electrolyte of this invention can suppress gas generation and DCIR impedance growth during long-cycle storage of the battery, and while improving high-voltage cycle performance, it also takes into account the low-temperature performance of the battery.

Description

Technical Field

[0001] This invention relates to the field of electrolytes, and particularly to an electrolyte and lithium-ion battery that suppress gas generation and DCIR internal resistance growth during long-cycle storage of lithium-ion batteries. Background Technology

[0002] With the continuous development of lithium-ion secondary batteries, higher requirements have been placed on the high voltage and high temperature resistance of electrolytes. Currently, there are two main methods to achieve high voltage and high temperature resistance of electrolytes: 1) improving the oxidation resistance of the solvent; 2) using high-temperature stable positive electrode additives, which can preferentially undergo oxidation reactions before the solvent, forming a passivation film on the positive electrode surface, inhibiting the dissolution of transition metal ions, and preventing the electrolyte from reacting with the electrode.

[0003] Patent JP2014063733A discloses that 2,4,6-tris(allyloxy)-1,3,5-triazine, when used as an additive, can improve the high-temperature cycle performance of lithium batteries under high voltage. Patent KR1020150032138A discloses a combined additive of triazine and lithium bis(lithium oxalate)borate; Patent KR1020150032139A discloses a combined additive of triazine and lithium difluorobis(oxalate)phosphate; and Patent KR1020150032140A discloses a combined additive of triazine and lithium difluorophosphate. All three combined additives can suppress gas generation during high-temperature storage of lithium batteries under high voltage. Therefore, triazine additives are high-performance, high-voltage, and high-temperature resistant additives that, when used in lithium-ion battery electrolytes, can improve the high-temperature cycle performance and high-temperature storage performance of lithium-ion batteries.

[0004] However, research by this invention has revealed that while the aforementioned triazine additives can indeed suppress gas production during the initial stages of battery storage at high voltage, gas production increases dramatically over longer periods (≥28 days), affecting the battery's electrochemical and safety performance. Researchers typically only examine high-temperature storage performance over 7–28 days, avoiding long-term testing, thus failing to identify the gas production defect during long-term high-temperature storage. Furthermore, the triazine additives exhibit high film-forming resistance at the negative electrode, impacting the battery's low-temperature and power performance.

[0005] Based on the need for long-cycle battery storage and the requirements for overall battery performance, it is necessary to develop an electrolyte composition or electrolyte formulation that can suppress gas generation during long-cycle storage at high temperatures while ensuring the battery's high and low temperature performance. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention proposes a lithium-ion battery electrolyte that suppresses gas generation during long-cycle battery storage, ensures high-voltage cycle performance, and significantly improves the high and low temperature performance of the battery.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] A lithium-ion battery electrolyte comprises a main lithium salt and an organic solvent, and the electrolyte further comprises:

[0009] The first additive is a triazine compound with the structure shown in formula (A):

[0010]

[0011] In the formula, R1, R2, and R3 are independently selected from C1-C6 alkyl, C1-C6 fluoroalkyl, C3-C6 cycloalkyl, C2-C6 alkenyl, C2-C6 alkynyl, or cyanoalkyl;

[0012] The second additive is lithium difluorophosphate-based fluoroborate or lithium difluorophosphate-based fluorophosphate with the structure shown in formula (B):

[0013]

[0014] In the formula, M is boron or phosphorus; when M is boron, x is selected from 1, 2, 3 or 4, y is selected from 0, 1, 2 or 3, and x + y = 4; when M is phosphorus, x is selected from 1, 2, 3, 4, 5 or 6, y is selected from 0, 1, 2, 3, 4 or 5, and x + y = 6;

[0015] The first additive accounts for 0.01 to 5.0 wt% of the total mass of the electrolyte;

[0016] The second additive accounts for 0.01 to 10.0 wt% of the total mass of the electrolyte.

[0017] Preferably, in formula (A), R1, R2, and R3 are independently selected from C1-C3 alkyl, C1-C3 fluoroalkyl, C3-C5 cycloalkyl, C2-C3 alkenyl, C2-C3 alkynyl, or cyanoalkyl.

[0018] More preferably,

[0019] The first additive is selected from at least one of the following structures:

[0020]

[0021] The second additive is selected from at least one of the following structures:

[0022]

[0023] The research conducted by this invention revealed that when the first additive is used alone, it results in poor film density and decreased thermal stability during long-term storage (≥28 days) at 60°C, with a sharp increase in gas production after 28 days of storage. When the second additive is used alone, it increases the acidity of the electrolyte, leading to poor stability of the positive electrode interface during long-term battery storage. Therefore, both the first and second additives of this invention exhibit poor stability during long-term high-temperature storage when used alone. However, this invention, through the combined use of a first additive and a second additive, allows the first additive to deposit components containing -CN=C- bonds on the positive and negative electrode surfaces during electrochemical decomposition during battery charging and discharging (as determined by XPS analysis of the CEI and SEI film components after battery formation). This component helps suppress the decomposition reaction of solvent and lithium salt at the positive and negative electrode interfaces, improving the battery's cycle performance. The second additive can form inorganic components containing P or B on the positive electrode surface to modify the N-containing organic components deposited on the surface by the first additive, ultimately forming an inorganic-organic composite interface film. This composite interface film has high stability and low film-forming impedance, effectively reducing the occurrence of interfacial side reactions and improving the battery's high-temperature cycle performance and low-temperature performance. Simultaneously, the first additive can effectively suppress the increase in acidity in the electrolyte, reduce the dissolution of high-voltage transition metal ions, and improve the stability of the positive electrode interface, thereby enhancing the battery's high-temperature storage performance.

[0024] The amount of the first additive added in this invention will have different effects on the application effect. When the content of the first additive is low (e.g., <0.01wt%), its effect on improving the cycle stability of the electrolyte is not obvious, and its effect on reducing HF content and improving the stability of the positive electrode interface is also not obvious. When the content of the first additive is high (e.g., >5.0wt%), its effect on improving the stability of the positive electrode interface is no longer enhanced, and the negative electrode interface impedance continues to increase, leading to the negative effect of low-temperature degradation. Therefore, preferably, the first additive accounts for 0.01 to 5.0wt% of the total mass of the electrolyte, more preferably 0.1 to 1.0wt%.

[0025] The amount of the second additive added in this invention will have different effects on the application effect. When the content of the second additive is low (e.g., <0.01wt%), its effect on reducing negative electrode impedance and negative electrode polarization is not significant; when the content of the second additive is high (e.g., >10.0wt%), its effect on reducing negative electrode impedance and negative electrode polarization will not continue to increase, and may also lead to a decrease in electrolyte conductivity and an increase in electrolyte acidity, thereby increasing the risk of gas generation during battery storage. Therefore, preferably, the second additive accounts for 0.1 to 10wt% of the total mass of the electrolyte, more preferably 0.1 to 3.0wt%.

[0026] The electrolyte of the present invention further includes a third additive, which is a nitrile additive selected from at least one of succinic anionibacterium, adiponitrile, methylglutaronitrile, ethylene glycol dipropionitrile ether, trans-butenedionitrile, hex-2-enidonitrile, di(2-cyanoethyl) sulfone, 1,3,6-hexanetricarbonitrile, 1,3,5-pentanetricarbonitrile, 1,2,3-tris(2-cyanoethoxy)propane, di(cyanoethyl) sulfate, tri(cyanoethyl) phosphate, or tri(cyanoethyl) borate; the amount of the third additive is 1.0 to 10.0 wt% of the total mass of the electrolyte.

[0027] Preferably, the third additive is selected from at least one of hexadionitrile, hex-2-enidonitrile, 1,3,6-hexanetricarbonyl, 1,2,3-tris(2-cyanoethoxy)propane, and tri(cyanoethyl) borate, and is used in an amount of 2.0 to 6.0 wt% of the total mass of the electrolyte, preferably 3.0 to 5.0 wt%.

[0028] In a preferred embodiment, the electrolyte contains 0.1 to 2.5 wt% of a first additive, 0.1 to 5.0 wt% of a second additive, and 2.0 to 6.0 wt% of a third additive.

[0029] In a more preferred embodiment, the electrolyte contains 0.2 to 0.5 wt% of a first additive, 0.2 to 1.0 wt% of a second additive, and 3.0 to 5.0 wt% of a third additive.

[0030] To further improve the overall performance of the battery, the electrolyte also includes a basic additive, which is selected from at least one of sulfur-containing oxygen double bond compounds, carbonate compounds, fluorine-containing lithium salt compounds, or cyclic ether compounds.

[0031] The sulfur-containing oxygen double bond compound is selected from at least one of 1,3-propanesulfonyl lactone, propenyl-1,3-sulfonyl lactone, 1,4-butanesulfonyl lactone, methylene disulfonate, vinyl sulfate, methyl vinyl sulfate, monofluorovinyl sulfate, or 4,4'-divinyl sulfate.

[0032] The carbonate compound is selected from at least one of fluoroethyl carbonate, difluoroethylene carbonate, trifluoromethyl propylene carbonate, vinylene carbonate, or vinyl ethylene carbonate.

[0033] The fluorinated lithium salt compound is selected from at least one of lithium difluorophosphate, lithium difluorooxalate borate, lithium bis(trifluoromethylsulfonyl)imide, lithium fluorosulfonate, lithium tetrafluorooxalate phosphate, lithium difluorobis(oxalate) phosphate, lithium tetrafluoroborate, or lithium tri(oxalate) phosphate.

[0034] The cyclic ether compounds are selected from at least one of 1,3-dioxane, 1,4-dioxane, 1,3-dioxolane, 2-methyl-1,3-dioxolane, or 4-methyl-1,3-dioxolane.

[0035] The amount of the basic additive is 0.1 to 6.0 wt% of the total mass of the electrolyte.

[0036] The main lithium salt of this invention can be any common lithium salt found in electrolytes. Preferably, the main lithium salt is selected from at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide, and is used in an amount of 8-20 wt% of the total mass of the electrolyte.

[0037] The organic solvents described in this invention include fluorocarbonate solvents and fluorocarboxylic acid ester solvents;

[0038] The fluorocarbonate solvent is selected from at least one of fluoroethylene carbonate, difluoroethylene carbonate, methyl trifluoroethyl carbonate, methyl difluoroethyl carbonate, bis(2,2,2-trifluoroethyl) carbonate, and 3,3,3-trifluoropropylene carbonate.

[0039] The fluorocarboxylic acid ester solvent is selected from at least one of ethyl 2,2-difluoroacetate, ethyl 2,2,2-trifluoroacetate, ethyl 2,2-difluoropropionate, and propyl 2,2-difluoropropionate.

[0040] The present invention also provides a lithium-ion battery, comprising a positive electrode, a negative electrode, a separator, and any of the electrolytes described above.

[0041] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0042] 1. By adding a first additive and a second additive, this invention eliminates the increased gas generation during long-term high-temperature storage caused by the low thermal stability of the first additive as a single organic film component under long-term conditions, and the gas generation during high-temperature storage caused by the decrease in positive electrode interface stability due to the increase in electrolyte acidity caused by the second additive. This improves the high-voltage cycle performance while also taking into account the high and low temperature performance of the battery.

[0043] 2. This invention further improves the storage and cycle performance of lithium batteries under high voltage by using the combination of the first additive, the second additive and the third additive. The highly electronegative CN in the third additive can form CN-Co bonds with the surface of positive electrode metal oxides such as LCO and NMC, inhibiting the direct contact of the electrolyte with the positive electrode, thereby improving the thermal stability of the high voltage positive electrode / electrolyte interface.

[0044] 3. This invention, through the combined use of a first additive, a second additive, a third additive, a basic additive containing S=O double bonds, and a fluorinated solvent, can further improve high-temperature stability at high voltages (>4.5V). The sulfur-containing additive can further form a highly oxidation-resistant composite organic film on the positive electrode surface. In the event of lithium plating at the negative electrode, it reduces lithium metal to form a protective film, thereby inhibiting the reaction with the electrolyte caused by lithium plating, which would otherwise degrade battery performance. The use of the fluorinated solvent can further enhance the electrochemical window at 4.5V vs. graphite, improving cycle performance at high voltages. Detailed Implementation

[0045] The present invention will be further described below with reference to specific embodiments, but the invention is not limited to these specific embodiments. Those skilled in the art should recognize that the present invention covers all alternatives, improvements, and equivalents that may be included within the scope of the claims.

[0046] I. Preparation of high-voltage lithium cobalt oxide electrolyte

[0047] Example 1

[0048] In an argon-filled glove box (moisture < 5 ppm, oxygen < 10 ppm), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and propyl propionate were uniformly mixed in a mass ratio of EC:PC:DEC:PP = 1.5:1.5:3:4. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution to dissolve it, preparing a lithium hexafluorophosphate solution. Then, 0.4% (based on the total mass of the electrolyte) of compound A1 and 0.5% (based on the total mass of the electrolyte) of compound B1 were added to the lithium hexafluorophosphate solution, and the mixture was stirred until homogeneous, resulting in a lithium hexafluorophosphate concentration of 1.0 M (approximately 12.5% ​​of the total electrolyte mass), thus obtaining the electrolyte of Example 1.

[0049] Example 2

[0050] In an argon-filled glove box (moisture < 5 ppm, oxygen < 10 ppm), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and propyl propionate (PP) were uniformly mixed in a mass ratio of EC:PC:DEC:PP = 1.5:1.5:3:4. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution to dissolve it, preparing a lithium hexafluorophosphate solution. Then, 0.4% (based on the total mass of the electrolyte) of compound A1, 0.5% (based on the total mass of the electrolyte) of compound B1, and 3% (based on the total mass of the electrolyte) of 1,3,6-hexanetrionitrile (HTCN) were added to the lithium hexafluorophosphate solution, and the mixture was stirred until homogeneous, resulting in a lithium hexafluorophosphate concentration of 1.0 M (approximately 12.5% ​​of the total electrolyte mass), yielding the electrolyte of Example 2.

[0051] Example 3

[0052] In an argon-filled glove box (moisture < 5 ppm, oxygen < 10 ppm), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), propyl propionate (PP), and fluoroethylene carbonate (FEC) were uniformly mixed at a mass ratio of EC:PC:DEC:PP:FEC = 1:1.5:3:4:0.5. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution to dissolve it, preparing a lithium hexafluorophosphate solution. Then, 0.4% (based on the total mass of the electrolyte) of compound A1, 0.5% (based on the total mass of the electrolyte) of compound B1, and 3% (based on the total mass of the electrolyte) of 1,3,6-hexanetrionitrile (HTCN) were added to the lithium hexafluorophosphate solution, and the mixture was stirred until homogeneous, resulting in a lithium hexafluorophosphate concentration of 1.0 M (approximately 12.5% ​​of the total electrolyte mass), yielding the electrolyte of Example 3.

[0053] Example 4

[0054] In an argon-filled glove box (moisture < 5 ppm, oxygen < 10 ppm), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl 2,2-difluoroacetate (DFEA), and fluoroethylene carbonate (FEC) were uniformly mixed in a mass ratio of EC:PC:DEC:DFEA:FEC = 1:1.5:3:4:0.5. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution to dissolve it, thus preparing a solution containing lithium hexafluorophosphate. Subsequently, 0.4% of compound A1, 0.5% of compound B1, 3% of 1,3,6-hexanetrionitrile (HTCN), and 1% of 1,2,3-tris(2-cyanoethoxy)propane (TCP) based on the total mass of the electrolyte were added to the solution containing lithium hexafluorophosphate. The mixture was stirred until homogeneous, resulting in a lithium hexafluorophosphate concentration of 1.0 M (approximately 12.5% ​​of the total mass of the electrolyte), thus obtaining the electrolyte of Example 4.

[0055] Example 5

[0056] In an argon-filled glove box (moisture < 5 ppm, oxygen < 10 ppm), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl 2,2-difluoroacetate (DFEA), and fluoroethylene carbonate (FEC) were uniformly mixed in a mass ratio of EC:PC:DEC:DFEA:FEC = 1:1.5:3:4:0.5. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution to dissolve it, thus preparing a solution containing lithium hexafluorophosphate. Subsequently, 0.4% of compound A1, 0.5% of compound B1, 3% of 1,3,6-hexanetrionitrile (HTCN), 1% of 1,2,3-tris(2-cyanoethoxy)propane (TCP), and 2% of 1,3-propanesulfonyl lactone (PS) based on the total mass of the electrolyte were added to the solution containing lithium hexafluorophosphate. The mixture was stirred until homogeneous, resulting in a lithium hexafluorophosphate concentration of 1.0 M (approximately 12.5% ​​of the total electrolyte mass), thus obtaining the electrolyte of Example 5.

[0057] Example 6

[0058] The electrolyte of Example 6 was obtained by replacing 0.4% A1 with 0.4% A2 in Example 3, while keeping the other parameters and preparation method the same as in Example 3.

[0059] Example 7

[0060] The electrolyte of Example 7 was obtained by replacing 0.4% A1 with 0.4% A5 in Example 3, while keeping the other parameters and preparation method the same as in Example 3.

[0061] Example 8

[0062] The electrolyte of Example 8 was obtained by replacing 0.4% A1 with 0.4% A7 in Example 3, while keeping the other parameters and preparation method the same as in Example 3.

[0063] Example 9

[0064] The electrolyte of Example 9 was obtained by replacing 0.4% A1 with 0.4% A9 in Example 3, while keeping the other parameters and preparation method the same as in Example 3.

[0065] Example 10

[0066] The electrolyte of Example 10 was obtained by replacing 0.4% Al with 0.2% Al in Example 3, while keeping the other parameters and preparation method the same as in Example 3.

[0067] Example 11

[0068] The electrolyte of Example 11 was obtained by replacing 0.4% Al with 0.2% Al in Example 5, while keeping the other parameters and preparation method the same as in Example 3.

[0069] Example 12

[0070] The electrolyte of Example 12 was obtained by replacing 0.4% Al with 1% Al in Example 3, while keeping the other parameters and preparation method the same as in Example 3.

[0071] Example 13

[0072] The electrolyte of Example 13 was obtained by replacing 0.4% A1 with 1% A9 in Example 3, with the remaining parameters and preparation method the same as in Example 3.

[0073] Example 14

[0074] The electrolyte of Example 14 was obtained by replacing 0.5% B1 with 0.5% B2 in Example 3, while keeping the other parameters and preparation method the same as in Example 3.

[0075] Example 15

[0076] The electrolyte of Example 15 was obtained by replacing 0.5% B1 in Example 3 with 1% B2, while keeping the other parameters and preparation method the same as in Example 3.

[0077] Example 16

[0078] The electrolyte of Example 16 was obtained by replacing 0.5% B1 in Example 3 with 0.2% B2, while keeping the other parameters and preparation method the same as in Example 3.

[0079] Example 17

[0080] The electrolyte of Example 17 was obtained by replacing 0.5% B1 in Example 3 with 0.2% B1, while keeping the other parameters and preparation method the same as in Example 3.

[0081] Example 18

[0082] The electrolyte of Example 18 was obtained by replacing 0.5% B1 in Example 3 with 1% B1, while keeping the other parameters and preparation method the same as in Example 3.

[0083] Example 19

[0084] The electrolyte of Example 19 was obtained by replacing 0.5% B1 in Example 3 with 2% B1, while keeping the other parameters and preparation method the same as in Example 1.

[0085] Example 20

[0086] The electrolyte of Example 20 was obtained by replacing 0.5% B1 in Example 3 with 5% B1, while keeping the other parameters and preparation method the same as in Example 3.

[0087] Example 21

[0088] The electrolyte of Example 21 was obtained by replacing 3% of 1,3,6-hexanetrionitrile (HTCN) in Example 3 with 3% methylglutaronitrile (MGN), while keeping the other parameters and preparation method the same as in Example 3.

[0089] Example 22

[0090] The electrolyte of Example 22 was obtained by replacing 0.4% Al with 2.5% Al in Example 3, while keeping the other parameters and preparation method the same as in Example 3.

[0091] Example 23

[0092] The electrolyte of Example 23 was obtained by replacing 0.4% Al with 0.1% Al in Example 3, while keeping the other parameters and preparation method the same.

[0093] Example 24

[0094] The electrolyte of Example 24 was obtained by replacing 3% of 1,3,6-hexanetrionitrile (HTCN) in Example 3 with 6% HTCN, while keeping the other parameters and preparation method the same as in Example 3.

[0095] Example 25

[0096] The electrolyte of Example 25 was obtained by replacing 3% of 1,3,6-hexanetrionitrile (HTCN) in Example 3 with 2% HTCN, while keeping the other parameters and preparation method the same as in Example 3.

[0097] Example 26

[0098] The electrolyte of Example 26 was obtained by replacing 0.5% B1 in Example 3 with 0.1% B1, while keeping the other parameters and preparation method the same as in Example 3.

[0099] Example 27

[0100] In an argon-filled glove box (moisture < 5 ppm, oxygen < 10 ppm), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl 2,2-difluoroacetate (DFEA), and fluoroethylene carbonate (FEC) were uniformly mixed in a mass ratio of EC:PC:DEC:DFEA:FEC = 1:1.5:3:4:0.5. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution to dissolve it, thus preparing a solution containing lithium hexafluorophosphate. Subsequently, 0.5% of compound A1, 0.2% of compound B1, 3% of 1,3,6-hexanetrionitrile (HTCN), 1% of 1,2,3-tris(2-cyanoethoxy)propane (TCP), and 1% of adiponitrile (ADN) and 2% of 1,3-propanesulfonyl lactone (PS) based on the total mass of the electrolyte were added to the solution containing lithium hexafluorophosphate. The mixture was stirred until homogeneous, resulting in a lithium hexafluorophosphate concentration of 1.0 M (approximately 12.5% ​​of the total mass of the electrolyte), thus obtaining the electrolyte of Example 27.

[0101] Comparative Example 1

[0102] In an argon-filled glove box (moisture < 5 ppm, oxygen < 10 ppm), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and propyl propionate were uniformly mixed in a mass ratio of EC:PC:DEC:PP = 1.5:1.5:3:4. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution to dissolve it, resulting in a lithium hexafluorophosphate concentration of 1.0 M (approximately 12.5% ​​of the total electrolyte mass), thus obtaining the electrolyte of Comparative Example 1.

[0103] Comparative Example 2

[0104] In an argon-filled glove box (moisture < 5 ppm, oxygen < 10 ppm), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and propyl propionate were uniformly mixed in a mass ratio of EC:PC:DEC:PP = 1.5:1.5:3:4. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution for dissolution. Then, 1,3,6-hexanetrionitrile (HTCN) was added at 3% of the total electrolyte mass, and the mixture was stirred until homogeneous. The resulting electrolyte had a lithium hexafluorophosphate concentration of 1.0 M (approximately 12.5% ​​of the total electrolyte mass), thus obtaining the electrolyte of Comparative Example 2.

[0105] Comparative Example 3

[0106] In an argon-filled glove box (moisture < 5 ppm, oxygen < 10 ppm), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), propyl propionate (PP), and fluoroethylene carbonate (FEC) were uniformly mixed at a mass ratio of EC:PC:DEC:PP:FEC = 1:1.5:3:4:0.5. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution to dissolve it, preparing a lithium hexafluorophosphate solution. Then, 1,3,6-hexanetrionitrile (HTCN) was added to the lithium hexafluorophosphate solution at 3% of the total electrolyte mass, and the mixture was stirred until homogeneous, resulting in a lithium hexafluorophosphate concentration of 1.0 M (approximately 12.5% ​​of the total electrolyte mass), yielding the electrolyte of Comparative Example 3.

[0107] Comparative Example 4

[0108] In an argon-filled glove box (moisture < 5 ppm, oxygen < 10 ppm), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl 2,2-difluoroacetate (DFEA), and fluoroethylene carbonate (FEC) were uniformly mixed at a mass ratio of EC:PC:DEC:DFEA:FEC = 1:1.5:3:4:0.5. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution to dissolve it, preparing a lithium hexafluorophosphate solution. Then, 3% (based on the total mass of the electrolyte) of 1,3,6-hexanetrionitrile (HTCN), 1% (based on the total mass of the electrolyte) of 1,2,3-tris(2-cyanoethoxy)propane (TCP), and 2% (based on the total mass of the electrolyte) of 1,3-propanesulfonyl lactone (PS) were added to the lithium hexafluorophosphate solution, and the mixture was stirred until homogeneous, yielding a lithium hexafluorophosphate concentration of 1.0 M (approximately 12.5% ​​of the total electrolyte mass), which is the electrolyte of Comparative Example 4.

[0109] Comparative Example 5

[0110] In an argon-filled glove box (moisture < 5 ppm, oxygen < 10 ppm), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), propyl propionate (PP), and fluoroethylene carbonate (FEC) were uniformly mixed at a mass ratio of EC:PC:DEC:PP:FEC = 1:1.5:3:4:0.5. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution to dissolve it, preparing a lithium hexafluorophosphate solution. Then, compound A1 (0.4% of the total electrolyte mass) was added to the lithium hexafluorophosphate solution, and the mixture was stirred until homogeneous, resulting in a lithium hexafluorophosphate concentration of 1.0 M (approximately 12.5% ​​of the total electrolyte mass), yielding the electrolyte of Comparative Example 5.

[0111] Comparative Example 6

[0112] In an argon-filled glove box (moisture < 5 ppm, oxygen < 10 ppm), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), propyl propionate (PP), and fluoroethylene carbonate (FEC) were uniformly mixed at a mass ratio of EC:PC:DEC:PP:FEC = 1.0:1.5:3:4:0.5. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution to dissolve it, preparing a lithium hexafluorophosphate solution. Then, compound B1 (0.5% of the total electrolyte mass) was added to the lithium hexafluorophosphate solution, and the mixture was stirred until homogeneous, resulting in a lithium hexafluorophosphate concentration of 1.0 M (approximately 12.5% ​​of the total electrolyte mass), yielding the electrolyte of Comparative Example 6.

[0113] Comparative Example 7

[0114] In an argon-filled glove box (moisture < 5 ppm, oxygen < 10 ppm), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and propyl propionate were uniformly mixed in a mass ratio of EC:PC:DEC:PP = 1.5:1.5:3:4. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution to dissolve it, preparing a lithium hexafluorophosphate solution. Then, 0.4% of compound A1 based on the total mass of the electrolyte and 3% of HTCN based on the total mass of the electrolyte were added to the lithium hexafluorophosphate solution, and the mixture was stirred until homogeneous, yielding a lithium hexafluorophosphate concentration of 1.0 M (approximately 12.5% ​​of the total electrolyte mass), thus obtaining the electrolyte of Comparative Example 7.

[0115] Comparative Example 8

[0116] In an argon-filled glove box (moisture < 5 ppm, oxygen < 10 ppm), ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), propyl propionate (PP), and fluoroethylene carbonate (FEC) were uniformly mixed at a mass ratio of EC:PC:DEC:PP:FEC = 1:1.5:3:4:0.5. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution to dissolve it, preparing a lithium hexafluorophosphate solution. Then, 0.5% of compound B1 (based on the total mass of the electrolyte) and 3% of HTCN (based on the total mass of the electrolyte) were added to the lithium hexafluorophosphate solution, and the mixture was stirred until homogeneous, yielding a lithium hexafluorophosphate concentration of 1.0 M (approximately 12.5% ​​of the total electrolyte mass), thus obtaining the electrolyte of Comparative Example 8.

[0117] The electrolyte formulations for each comparative example and embodiment are shown in Table 1 below:

[0118] Table 1. Types and amounts of additives in each example and comparative example.

[0119]

[0120]

[0121] II. LCO / Graphite Battery Fabrication and Performance Testing

[0122] The lithium-ion battery electrolytes of the above embodiments and comparative examples were respectively loaded into a 1000mAh soft-pack lithium-ion battery. The lithium-ion power battery includes a positive electrode, a negative electrode, a separator, an electrolyte, and battery auxiliary materials. The positive electrode active material is LiCoO2, and the negative electrode active material is graphite.

[0123] The preparation process is as follows: the positive electrode, separator and negative electrode are wound together into a core, sealed with aluminum-plastic film and then baked to ensure that the electrode moisture meets the requirements. After baking, the cell is injected with electrolyte and then subjected to standing, formation, capacity testing and aging processes to obtain the finished soft-pack cell.

[0124] The performance of the aforementioned lithium-ion batteries was tested, including:

[0125] (1) Cyclic performance test

[0126] Charge the battery at a constant current of 1C to the charging cutoff voltage at a specified temperature, then charge at a constant voltage until the current drops to 0.1C, and then discharge at a constant current of 1C to 3.0V. Repeat this cycle for a specific number of cycles, recording the discharge capacity of the first cycle and the discharge capacity of the last cycle. Calculate the battery's capacity retention rate using the following formula:

[0127] Capacity retention rate = discharge capacity in the last week / discharge capacity in the first week * 100%.

[0128] (2) High-temperature long-cycle storage performance test

[0129] The battery was charged at a constant current of 1C at room temperature until the charging cutoff voltage, and then charged at a constant voltage until the current dropped to 0.1C. It was then stored in a 60°C oven for 42 days. The volume and DCIR internal resistance before and after high-temperature storage were recorded. The volume expansion rate after storage was calculated using the following formula:

[0130] Volume expansion rate = (volume after storage - volume before storage) / volume before storage * 100%.

[0131] DCIR internal resistance = (DCIR impedance after storage - DCIR impedance before storage) / DCIR impedance before storage * 100%

[0132] (3) Low-temperature discharge

[0133] Low-temperature discharge test: At room temperature, charge at a constant current of 0.5C to the charging cutoff voltage, then charge at a constant voltage until the current drops to 0.1C, and then discharge at a constant current of 0.5C to 3.0V at a specified temperature. Record the discharge capacity at the lowest temperature.

[0134] Low-temperature capacity retention rate = Discharge capacity at low temperature / Discharge capacity in week 1 * 100%.

[0135] The results of the above electrochemical performance tests are shown in Table 2 below:

[0136] Table 2 Electrochemical test results of LCO / AG from -4.5V to 3.0V

[0137]

[0138]

[0139] According to the test results in Table 2 above:

[0140] Comparing Example 1, Comparative Example 1, Comparative Example 5 and Comparative Example 6, it can be seen that the combined use of Compound A1 and Compound B1, compared with their individual use, can suppress gas generation and internal resistance growth after 42 days of long-cycle storage in the lithium cobalt oxide high-voltage system, improve long-cycle storage stability, and at the same time improve the high-voltage cycle performance of the battery while taking into account low-temperature performance.

[0141] Compared with Examples 2-5, the combination of compound A1, compound B1 and the third type of nitrile additive has better high voltage resistance, specifically in terms of improved high voltage cycle life and improved high voltage storage stability.

[0142] Compared to Examples 3 and 14, the structure of compound B1 exhibits superior performance in both high and low temperatures compared to the structure of compound B2. Compared to Examples 3 and 6-9, compound A1 exhibits the best performance in both high and low temperatures.

[0143] Compared with Examples 3, 18, 19, 20, and 26, compound B1 showed good performance in the concentration range of 0.1% to 5%. With the increase of additives, the risk of gas generation during high-temperature storage increased slightly. When the additive was reduced to 0.1%, the effect of reducing impedance and improving low-temperature performance deteriorated slightly. Compared with Examples 3, 12, and 22, compound A1 showed good performance in the concentration range of 2.5%. When the content exceeded 1%, the improvement in cycle performance was not significant, and the battery's low-temperature performance and the effect of suppressing gas generation during long-term storage deteriorated slightly.

[0144] Comparing Examples 3 and 21, when the third additive was replaced by MGN instead of HTCN, the battery's low-temperature performance improved, but its high-voltage long-cycle and high-temperature storage performance deteriorated. Comparing Examples 3 and 4, 24, and 25, the third additive showed some effect at concentrations between 2% and 6%. Increasing the third additive content from 4% to 6% slightly deteriorated the low-temperature performance, while decreasing it from 4% to 2% slightly deteriorated the high-temperature performance.

[0145] Compared with Examples 1 and 3, 4 and 5, the present invention, through the combined use of a first additive, a second additive, a third additive, a basic additive containing an S=O double bond, and a fluorinated solvent, can further improve the high-voltage cycle stability and high-temperature stability at 4.5V, while also taking into account low-temperature performance.

[0146] III. Preparation of NCM / graphite electrolyte system

[0147] Example 28:

[0148] In an argon-filled glove box (moisture < 5 ppm, oxygen < 10 ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were uniformly mixed at a mass ratio of EC:EMC:DEC = 3:5:2. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution to dissolve it, preparing a lithium hexafluorophosphate solution. Then, 0.2 wt% of compound A1 and 1.0 wt% of compound B1 were added to the lithium hexafluorophosphate solution to obtain the electrolyte of this example. After mixing and shaking, the concentration of lithium hexafluorophosphate in the lithium-ion battery electrolyte was 1.0 M, resulting in the electrolyte of Example 28.

[0149] Example 29:

[0150] The electrolyte of Example 29 was obtained by replacing 1 wt% B1 with 1 wt% B2 in Example 28, while keeping the other parameters and preparation method the same as in Example 28.

[0151] Example 30:

[0152] The electrolyte of Example 30 was obtained by replacing 0.2 wt% Al with 0.5 wt% Al in Example 28, while keeping the other parameters and preparation method the same as in Example 28.

[0153] Example 31:

[0154] The electrolyte of Example 31 was obtained by replacing 0.5 wt% A1 with 0.5 wt% A9 in Example 30, while keeping the other parameters and preparation method the same.

[0155] Example 32:

[0156] The electrolyte of Example 32 was obtained by replacing 1 wt% B1 with 2 wt% B1 in Example 28, while keeping the other parameters and preparation method the same as in Example 28.

[0157] Example 33

[0158] The electrolyte of Example 33 was obtained by replacing 1 wt% B1 in Example 28 with 3 wt% B1, while keeping the other parameters and preparation method the same as in Example 28.

[0159] Example 34

[0160] The electrolyte of Example 34 was obtained by replacing 1 wt% B1 in Example 28 with 0.1 wt% B1, while keeping the other parameters and preparation method the same as in Example 28.

[0161] Example 35

[0162] The electrolyte of Example 35 was obtained by replacing 0.2 wt% A1 with 0.2 wt% A6 in Example 28, while keeping the other parameters and preparation method the same.

[0163] Example 36

[0164] The electrolyte of Example 36 was obtained by replacing 0.5 wt% A1 with 0.5 wt% A8 in Example 30, while keeping the other parameters and preparation method the same.

[0165] Example 37

[0166] In an argon-filled glove box (moisture < 5 ppm, oxygen < 10 ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were uniformly mixed at a mass ratio of EC:EMC:DEC = 3:5:2. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution to dissolve it, preparing a lithium hexafluorophosphate solution. Then, 0.2 wt% of compound A1, 1.0 wt% of compound B1, and 1.0 wt% of ethylene sulfate (DTD) were added to the lithium hexafluorophosphate solution to obtain the electrolyte of this example. After mixing and shaking, the concentration of lithium hexafluorophosphate in the lithium-ion battery electrolyte was 1.0 M, resulting in the electrolyte of Example 37.

[0167] Comparative Example 9

[0168] In an argon-filled glove box (moisture < 5 ppm, oxygen < 10 ppm), ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were uniformly mixed in a mass ratio of EC:EMC:DEC = 3:5:2. Lithium hexafluorophosphate (LiPF6) was slowly added to the mixed solution to dissolve it, thus preparing a lithium hexafluorophosphate solution and obtaining the electrolyte of Comparative Example 9.

[0169] Comparative Example 10

[0170] 0.2 wt% Al was added to the electrolyte of Comparative Example 9, and the remaining parameters and preparation methods were the same as those of Comparative Example 9, to obtain the electrolyte of Comparative Example 10.

[0171] Comparative Example 11

[0172] Add 1 wt% B1 to the electrolyte of Comparative Example 9, and keep the other parameters and preparation method the same as Comparative Example 9 to obtain the electrolyte of Comparative Example 11.

[0173] Comparative Example 12

[0174] In the electrolyte of Comparative Example 9, 0.2 wt% Al and 1% lithium difluorophosphate (LiPO2F2) were added, and the remaining parameters and preparation methods were the same as those of Comparative Example 9, to obtain the electrolyte of Comparative Example 12.

[0175] Comparative Example 13

[0176] In the electrolyte of Comparative Example 9, 0.2 wt% Al and 1% lithium bis(fluorosulfonyl)imide (LiFSI) were added, and the remaining parameters and preparation methods were the same as those of Comparative Example 9, to obtain the electrolyte of Comparative Example 13.

[0177] Comparative Example 14

[0178] To the electrolyte of Comparative Example 9, 0.2 wt% of 1,3-propanesulfonyl lactone (PS) and 1 wt% B1 were added, with the remaining parameters and preparation method the same as in Comparative Example 9, to obtain the electrolyte of Comparative Example 14.

[0179] Table 3. Types and amounts of additives in each example and comparative example.

[0180]

[0181]

[0182] IV. Manufacturing and Performance Testing of NCM622 / Graphite Batteries

[0183] The lithium-ion battery electrolytes of the above embodiments and comparative examples were used to make soft-pack lithium-ion power batteries with a capacity of 1000mAh. The lithium-ion power battery includes a positive electrode, a negative electrode, a separator, an electrolyte, and battery auxiliary materials. The positive electrode active material is NCM622, and the negative electrode active material is graphite.

[0184] The preparation process is as follows: the positive electrode, separator and negative electrode are wound together into a core, sealed with aluminum-plastic film and then baked to ensure that the electrode moisture meets the requirements. After baking, the cell is injected with electrolyte and then subjected to standing, formation, capacity testing and aging processes to obtain the finished soft-pack cell.

[0185] The cycle and low-temperature discharge test methods, capacity conversion, and volume expansion rate test methods for the NCM622 / graphite battery system are the same as those for lithium cobalt oxide. The only change is that the charge / discharge voltage is adjusted to 2.8-4.35V. The high-temperature storage conditions for the NCM622 / graphite battery system are changed to 60℃ storage for 60 days (d), with a charge / discharge voltage of 2.8-4.35V. The volume measurement method is the same as that for lithium cobalt oxide.

[0186] The electrochemical performance comparisons are shown in Table 4:

[0187] Table 4 Electrochemical test results of NCM622 / AG from 4.35V to 2.8V

[0188]

[0189]

[0190] According to the results in Table 4 above, in Comparative Examples 28 and 10 and 11, the combined use of Compound A1 and Compound B1, compared to their individual use, can suppress gas generation and internal resistance growth after 60 days of long-cycle storage in the ternary system, improving long-cycle storage stability, while also enhancing the high-voltage cycle performance and maintaining low-temperature performance. In Comparative Examples 28 and 12, the combined use of Compound A1 and Compound B1 shows a slight advantage in high-temperature storage compared to the combination of Compound A1 and commonly used LiPO2F2. In Comparative Examples 28 and 13, the combined use of Compound A1 and Compound B1 outperforms LiFSI in overall performance compared to the combination of Compound A1 and commonly used LiFSI. In Comparative Examples 28 and 14, the combined use of Compound A1 and Compound B1 significantly improves all performance aspects, both at high and low temperatures, compared to the combination of commonly used PS and Compound B1.

Claims

1. A lithium ion battery electrolyte comprising a main lithium salt, an organic solvent, characterized in that: The electrolyte further includes: The first additive is selected from at least one of the following structures: The second additive is selected from at least one of the following structures: The first additive accounts for 0.1 to 2.5 wt% of the total mass of the electrolyte; The second additive accounts for 0.1 to 5.0 wt% of the total mass of the electrolyte.

2. The electrolyte for lithium ion batteries according to claim 1, characterized in that: The electrolyte further includes a third additive, which is a nitrile additive selected from at least one of the following: butadiene nitrile, adiponitrile, methylglutaronitrile, ethylene glycol dipropionitrile ether, trans-butenedionitrile, hex-2-enedionitrile, di(2-cyanoethyl) sulfone, 1,3,6-hexanetricarbonitrile, 1,3,5-pentanetricarbonitrile, 1,2,3-tris(2-cyanoethoxy)propane, di(cyanoethyl) sulfate, tri(cyanoethyl) phosphate, or tri(cyanoethyl) borate. The amount of the third additive is 1.0 to 10.0 wt% of the total mass of the electrolyte.

3. The lithium-ion battery electrolyte according to claim 2, characterized in that: The first additive accounts for 0.1 to 1.0 wt% of the total mass of the electrolyte; The second additive accounts for 0.1–3.0 wt% of the total mass of the electrolyte; The third additive accounts for 3.0 to 5.0 wt% of the total mass of the electrolyte.

4. The electrolyte for lithium ion batteries according to claim 1, characterized in that: The electrolyte also includes a basic additive, which is selected from at least one of sulfur-containing oxygen double bond compounds, carbonate compounds, fluorinated lithium salt compounds, or cyclic ether compounds; The sulfur-containing oxygen double bond compound is selected from at least one of 1,3-propanesulfonyl lactone, propenyl-1,3-sulfonyl lactone, 1,4-butanesulfonyl lactone, methylene disulfonate, vinyl sulfate, methyl vinyl sulfate, vinyl monofluorosulfate, or 4,4'-divinyl sulfate. The carbonate compound is selected from at least one of fluoroethyl carbonate, difluoroethylene carbonate, trifluoromethyl propylene carbonate, vinylene carbonate, or vinyl ethylene carbonate. The fluorinated lithium salt compound is selected from at least one of lithium difluorophosphate, lithium difluorooxalate borate, lithium bis(trifluoromethylsulfonyl)imide, lithium fluorosulfonate, lithium tetrafluorooxalate phosphate, lithium difluorobis(oxalate) phosphate, lithium tetrafluoroborate, or lithium tri(oxalate) phosphate. The cyclic ether compound is selected from at least one of 1,3-dioxane, 1,4-dioxane, 1,3-dioxolane, 2-methyl-1,3-dioxolane, or 4-methyl-1,3-dioxolane. The amount of the basic additive is 0.1 to 6.0 wt% of the total mass of the electrolyte.

5. The electrolyte for lithium ion batteries according to any one of claims 1 to 4, characterized in that: The main lithium salt is selected from at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethylsulfonyl)imide, and its amount accounts for 8 to 20 wt% of the total mass of the electrolyte.

6. The electrolyte for lithium ion batteries according to any one of claims 1 to 4, characterized in that: The organic solvents include fluorocarbonate solvents and fluorocarboxylic acid ester solvents; The fluorocarbonate solvent is selected from at least one of fluoroethylene carbonate, difluoroethylene carbonate, methyl trifluoroethyl carbonate, methyl difluoroethyl carbonate, bis(2,2,2-trifluoroethyl) carbonate, and 3,3,3-trifluoropropylene carbonate. The fluorocarboxylic acid ester solvent is selected from at least one of ethyl 2,2-difluoroacetate, ethyl 2,2,2-trifluoroacetate, ethyl 2,2-difluoropropionate, and propyl 2,2-difluoropropionate.

7. The lithium-ion battery electrolyte according to claim 6, characterized in that: The fluorocarbonate solvent accounts for 0.01 to 30 wt% of the total mass of the electrolyte; the fluorocarboxylic acid ester solvent accounts for 0.01 to 50 wt% of the total mass of the electrolyte.

8. A lithium-ion battery, comprising a positive electrode, a negative electrode, and a separator, characterized in that: The lithium-ion battery further includes the electrolyte according to any one of claims 1-7.

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