A sodium-ion battery

Abstract

This invention discloses a sodium-ion battery, comprising a positive electrode, a negative electrode, and an electrolyte. The electrolyte includes a sodium salt, a non-aqueous organic solvent, and additive I, which is a fluorinated cyclic triphosphazene compound. The negative electrode includes a negative current collector and a negative active material layer. The negative active material layer is coated on at least one surface of the negative current collector and includes a negative active material and additive II, which is a pyrazine compound. This sodium-ion battery introduces additive I (a fluorinated cyclic triphosphazene compound) into the electrolyte and simultaneously introduces additive II (a pyrazine compound) into the negative active material layer. The pyrazine ring of additive II interacts with the cyclic structure of additive I through an aromatic ring conjugation effect, enhancing the interaction force between the electrolyte and the negative electrode surface, thereby improving the wettability of the electrolyte. Simultaneously, a nitrogen-containing organic solid electrolyte interfacial film is generated, improving the interfacial mechanical strength and reducing interfacial impedance.

Description

Technical Field

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

[0002] Sodium-ion batteries, with their outstanding advantages such as low cost and excellent rate performance, are widely recognized as a promising next-generation energy storage device, showing broad application prospects in large-scale energy storage, new energy vehicles, and other fields. However, safety and cycle life issues urgently need to be addressed during their preparation and application.

[0003] The solvent components in the electrolyte used in sodium-ion batteries often include organic ester solvents. These solvents are flammable, making the electrolyte highly flammable under abnormal operating conditions such as high temperature and overcharging, which can cause the battery to catch fire or even explode, posing a serious threat to battery safety.

[0004] Currently, the industry commonly uses the addition of fluorinated triphosphazene compounds, such as ethoxy (pentafluoro)cyclotriphosphazene, to improve the flame retardant properties of electrolytes. These fluorinated triphosphazene compounds possess excellent flame retardant and high-voltage resistance properties, effectively reducing the self-extinguishing time of the electrolyte and inhibiting combustion reactions. However, this solution also brings new problems: to achieve the ideal flame retardant effect, a high content of fluorinated triphosphazene compounds needs to be added, which increases the viscosity of the electrolyte, making it difficult for the electrolyte to fully wet the electrodes, resulting in increased battery polarization; at the same time, the fluorinated triphosphazene compounds react inside the battery to generate sodium fluoride, which, although it can improve the mechanical strength of the solid electrolyte interphase (SEI) film, increases battery impedance, causing local rupture and regeneration of the SEI film, continuously consuming sodium ions, and ultimately shortening the battery's cycle life. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a sodium-ion battery.

[0006] This invention discloses a sodium-ion battery, comprising a positive electrode, a negative electrode, and an electrolyte; wherein the electrolyte comprises a sodium salt, a non-aqueous organic solvent, and additive I, additive I being a fluorinated cyclotriphosphazene compound; the negative electrode comprises a negative current collector and a negative active material layer, the negative active material layer being coated on at least one surface of the negative current collector, the negative active material layer comprising a negative active material and additive II, additive II being a pyrazine compound.

[0007] According to one embodiment of the present invention, the mass content of additive I in the electrolyte is: The mass content of additive II in the negative electrode active material layer is: , .

[0008] According to one embodiment of the present invention, the mass content of additive I in the electrolyte Mass content of Additive II in the negative electrode active material layer The following relationship must be satisfied: .

[0009] According to one embodiment of the present invention, the mass content of additive I in the electrolyte Mass content of Additive II in the negative electrode active material layer The following relationship must be satisfied: .

[0010] According to one embodiment of the present invention, additive I is selected from one or more of ethoxy(pentafluoro)cyclotriphosphazene, hexafluorocyclotriphosphazene and pentafluoro(phenoxy)cyclotriphosphazene.

[0011] According to one embodiment of the present invention, the mass content of additive I in the electrolyte The percentages are 2wt%, 3wt%, 4.5wt%, 7wt%, 8wt%, 9wt%, or 9.5wt%.

[0012] According to one embodiment of the present invention, additive II is selected from pyrazine compounds containing hydrocarbon groups.

[0013] According to one embodiment of the present invention, additive II is selected from one or more of 2,5-dibutylpyrazine, 2,5-di-tert-butylpyrazine and 2,5-diethoxypyrazine.

[0014] According to one embodiment of the present invention, the mass content of additive II in the negative electrode active material layer The values ​​are 150ppm, 300ppm, 700ppm, 1500ppm, 2100ppm, or 2500ppm.

[0015] According to one embodiment of the present invention, the negative electrode active material is selected from one or more of hard carbon, soft carbon, sodium titanate, metallic sodium, and metals that can form alloys with sodium.

[0016] Compared with the prior art, the sodium-ion battery of the present invention has the following advantages: The sodium-ion battery of the present invention introduces additive I, a fluorinated triphosphazene compound, into the electrolyte to improve flame retardant performance. Simultaneously, additive II, a pyrazine compound, is introduced into the negative electrode active material layer. The strong aromatic ring conjugation effect between the pyrazine ring of additive II and the cyclic structure of additive I enhances the interaction force between the electrolyte and the negative electrode surface, thereby improving the wettability of the electrolyte. Furthermore, additive II contains nitrogen, which can generate a nitrogen-containing organic solid electrolyte interfacial film. This solid electrolyte interfacial film contains organic nitrogen-containing sodium salts to improve the sodium fluoride-rich interface, increase interfacial mechanical strength, reduce interfacial impedance, optimize interfacial stability, and thus improve the cycle life of the sodium-ion battery. Detailed Implementation

[0017] The following will disclose specific embodiments of the present invention. For clarity, many practical details will be described in the following description. However, it should be understood that these practical details are not intended to limit the present invention. That is, in some embodiments of the present invention, these practical details are not essential.

[0018] This embodiment provides a sodium-ion battery, including a positive electrode, a negative electrode, and an electrolyte.

[0019] Specifically, the positive electrode includes a positive current collector and a positive active material layer, the positive active material layer being coated on at least one surface of the positive current collector. The positive active material layer includes a positive active material, a positive conductive agent, and a positive binder. The negative electrode includes a negative current collector and a negative active material layer, the negative active material layer being coated on at least one surface of the negative current collector. The negative active material layer includes a negative active material, a negative conductive agent, and a negative binder. The electrolyte includes a sodium salt and a non-aqueous organic solvent.

[0020] Furthermore, the electrolyte also includes additive I, whose mass content in the electrolyte is: Additive I is a fluorinated triphosphazene compound whose molecular structure contains phosphorus, nitrogen, and fluorine atoms, giving it excellent flame-retardant properties.

[0021] In this embodiment, additive I is selected from one or more of ethoxy(pentafluoro)cyclotriphosphazene (PFPN), hexafluorocyclotriphosphazene (HFPN), and pentafluoro(phenoxy)cyclotriphosphazene (FPPN).

[0022] In this embodiment, additive I is introduced into the electrolyte to improve its flame retardant properties. Additive I is a fluorinated triphosphazene compound, which, as a functional additive with both excellent flame retardant properties and high-pressure resistance, can effectively inhibit the combustion reaction by reducing the self-extinguishing time of the electrolyte.

[0023] However, excessive amounts of Additive I can increase the viscosity of the electrolyte system, weakening its wetting ability on the electrodes and thus exacerbating battery polarization, affecting electrochemical performance. Simultaneously, during battery charging and discharging, Additive I participates in electrochemical reactions and generates sodium fluoride. While the formation of sodium fluoride helps enhance the mechanical strength of the solid electrolyte interphase (SEI) film, it also increases the battery's interfacial impedance, making the SEI film prone to localized rupture and regeneration, continuously consuming active sodium ions, and ultimately leading to a decrease in battery cycle life.

[0024] To address this issue, this embodiment introduces additive II into the negative electrode active material layer of the negative electrode sheet.

[0025] Specifically, the negative electrode active material layer also includes additive II, the mass content of which in the negative electrode active material layer is: , Additive II is a pyrazine compound. Compared to other azabenzene compounds such as pyridine, pyrimidine, and pyridazine, pyrazine compounds are less prone to electrophilic substitution reactions but readily react with nucleophiles. Therefore, they easily undergo reduction reactions at the negative electrode, reducing polarization during battery formation and resulting in a more uniform distribution of the organic and sodium fluoride components in the SEI film. Preferably, additive II is a pyrazine compound containing a hydrocarbon group. Because the pyrazine ring contains a hydrocarbon group, it has an electron-donating effect, increasing the electron cloud density of the pyrazine compound, enhancing the interaction between additive I and additive II, increasing the wettability of the electrolyte on the negative electrode, and regulating the organic composition of the SEI film, thus reducing battery polarization.

[0026] In this embodiment, additive II is selected from one or more of 2,5-dibutylpyrazine, 2,5-di-tert-butylpyrazine and 2,5-diethoxypyrazine.

[0027] This embodiment utilizes the strong aromatic ring conjugation effect between the pyrazine ring of additive II and the cyclic structure of additive I to enhance the interaction force between the electrolyte and the negative electrode surface, thereby improving the wettability of the electrolyte. At the same time, additive II contains nitrogen, which can generate a nitrogen-containing organic solid electrolyte interfacial film. The solid electrolyte interfacial film contains organic nitrogen-containing sodium salt to improve the sodium fluoride interface, enhance the interfacial mechanical strength and reduce the interfacial impedance, optimize the interfacial stability, and thus improve the cycle life of the sodium-ion battery.

[0028] Since additive II is a solid, additive I will interact with it on the surface of additive II. Therefore, the mass content of additive I in the electrolyte... Mass content of Additive II in the negative electrode active material layer The following relationship must also be satisfied: This limits the surface content of additive II.

[0029] When the relationship is satisfied, the battery's discharge DC internal resistance, rate cycle capacity retention, high-temperature cycle capacity retention, and first-cycle coulombic efficiency can all reach a high level. When the ratio is too high, it leads to excessive deposition of sodium fluoride on the surface of the negative electrode, while the viscosity of the electrolyte system increases, exacerbating battery polarization and affecting ion transport kinetics; while when When the ratio is too low, additive II will form a steric hindrance effect, reducing the effective contact area between the electrolyte and the negative electrode active material, resulting in an increase in local current density, which in turn causes uneven deposition of sodium metal on the surface of the negative electrode, increasing the risk of sodium deposition and affecting the safety and cycle life of the battery.

[0030] In this embodiment, aluminum foil is used as the positive current collector, and NFM (NaNi) is used as the positive active material. 1 / 3 Fe 1 / 3 Mn 1 / 3O2), the positive electrode conductive agent is conductive carbon black (Super-P), and the positive electrode binder is polyvinylidene fluoride (PVDF).

[0031] In this embodiment, the negative electrode current collector is aluminum foil, the negative electrode active material is one or more of hard carbon, soft carbon, sodium titanate, metallic sodium, and metals that can form alloys with sodium, the negative electrode conductive agent is conductive carbon black (Super-P), and the negative electrode binder is styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC).

[0032] In this embodiment, the sodium salt is selected from one or more of sodium hexafluorophosphate, sodium perchlorate, sodium difluorooxalate borate, sodium bis(oxalate borate), sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethylsulfonyl)imide, and sodium trifluorosulfonate. The non-aqueous organic solvent is selected from dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, ethylene carbonate, propylene carbonate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, tetrahydrofuran, furan, and 1,3-difluorosulfonate. One or more of dioxapentanes.

[0033] To further illustrate the improved performance of the sodium-ion battery produced by the present invention, several sodium-ion battery samples are provided below for detailed explanation.

[0034] Sample Example 1 (A1) The sodium-ion battery provided in Sample Example 1 includes a positive electrode, a negative electrode, and an electrolyte. Details are as follows: The positive electrode includes a positive current collector and a positive active material layer, which is coated on both sides of the positive current collector. This positive active material layer consists of positive active material, a positive conductive agent, and a positive binder in a mass ratio of 94:3:3. The positive current collector is made of aluminum foil, and the positive active material is NFM (NaNiO).1 / 3 Fe 1 / 3 Mn 1 / 3 O2), the positive electrode conductive agent is conductive carbon black (Super-P), and the positive electrode binder is polyvinylidene fluoride (PVDF).

[0035] The negative electrode sheet includes a negative current collector and a negative active material layer, which is coated on both sides of the negative current collector. The negative active material layer is composed of negative active material, negative conductive agent, negative binder and additive II in a mass ratio of 93.75:2.5:3.5:0.25. Among them, the negative current collector is aluminum foil, the negative active material is hard carbon, the negative conductive agent is conductive carbon black (Super-P), the negative binder is styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) in a mass ratio of 2.5:1, and additive II is 2,5-dibutylpyrazine.

[0036] The electrolyte is composed of sodium salt, non-aqueous organic solvent and additive I in a mass ratio of 12:86:2; wherein, the sodium salt is sodium hexafluorophosphate, the non-aqueous organic solvent is ethylene carbonate, propylene carbonate and diethyl carbonate in a mass ratio of 75:15:10, and additive I is ethoxy(pentafluoro)cyclotriphosphazene (PFPN).

[0037] The sodium-ion battery fabrication method includes four processes: first, preparing the positive electrode; second, preparing the negative electrode; third, preparing the electrolyte; and fourth, preparing the sodium-ion battery. The specific fabrication methods are as follows: (1) Preparation of positive electrode sheet The positive electrode active material, positive electrode conductive agent and positive electrode binder are mixed and dispersed in N-methyl-2-pyrrolidone (NMP) according to the mass ratio to obtain the positive electrode slurry. The positive electrode slurry is then uniformly coated on both sides of the positive electrode current collector. After drying, rolling and vacuum drying, the positive electrode active material layer is formed on both sides of the positive electrode current collector. After aluminum lead wires are welded on with an ultrasonic welding machine, the positive electrode sheet is obtained.

[0038] (2) Preparation of negative electrode sheet The negative electrode active material, negative electrode conductive agent, negative electrode binder and additive II are mixed and dispersed in deionized water according to the mass ratio to obtain a negative electrode slurry. The negative electrode slurry is then coated on both sides of the negative electrode current collector. After drying, rolling and vacuum drying, a negative electrode active material layer is formed on both sides of the negative electrode current collector. Nickel leads are then welded on using an ultrasonic welding machine to obtain the negative electrode sheet.

[0039] (3) Preparation of electrolyte Ethylene carbonate, propylene carbonate, and diethyl carbonate are mixed in a certain mass ratio to obtain a non-aqueous organic solvent. Then, sodium salt, non-aqueous organic solvent, and additive I are mixed in a certain mass ratio to obtain an electrolyte.

[0040] (4) Preparation of sodium-ion batteries Three layers of separators are placed between the positive and negative electrode sheets. The sandwich structure consisting of the positive electrode sheet, negative electrode sheet, and separator is then wound to obtain a wound body. The wound body is then flattened and placed in an aluminum foil packaging bag, and vacuum baked at 85°C for 48 hours to obtain a battery cell. In a glove box with the dew point controlled below -40°C, the electrolyte is injected into the battery cell, which is then vacuum sealed and allowed to stand for 24 hours for formation to obtain a sodium-ion battery.

[0041] Sample Example 2 (A2) Compared to Sample Example 1, the sodium-ion battery provided in Sample Example 2 has a mass ratio of sodium salt, non-aqueous organic solvent, and additive I of 12:85:3.

[0042] Sample Example 3 (A3) Compared to Sample Example 2, the sodium-ion battery provided in Sample Example 3 has a mass ratio of negative electrode active material, negative electrode conductive agent, negative electrode binder and additive II of 93.79:2.5:3.5:0.21.

[0043] Sample Example 4 (A4) Compared to Sample Example 3, the sodium-ion battery provided in Sample Example 4 has a mass ratio of sodium salt, non-aqueous organic solvent and additive I of 12:83.5:4.5.

[0044] Sample Example 5 (A5) Compared to Sample Example 4, the sodium-ion battery provided in Sample Example 5 has a mass ratio of negative electrode active material, negative electrode conductive agent, negative electrode binder and additive II of 93.85:2.5:3.5:0.15.

[0045] Sample Example 6 (A6) Compared to Sample Example 5, the sodium-ion battery provided in Sample Example 6 has a mass ratio of sodium salt, non-aqueous organic solvent, and additive I of 12:81:7.

[0046] Sample Example 7 (A7) Compared to Sample Example 6, the sodium-ion battery provided in Sample Example 7 has a mass ratio of negative electrode active material, negative electrode conductive agent, negative electrode binder and additive II of 93.93:2.5:3.5:0.07.

[0047] Sample Example 8 (A8) Compared to Sample Example 7, the sodium-ion battery provided in Sample Example 8 has a mass ratio of sodium salt, non-aqueous organic solvent, and additive I of 12:80:8.

[0048] Sample Example 9 (A9) Compared to Sample Example 8, the sodium-ion battery provided in Sample Example 9 has a mass ratio of negative electrode active material, negative electrode conductive agent, negative electrode binder and additive II of 93.97:2.5:3.5:0.03.

[0049] Sample Example 10 (A10) Compared to Sample Example 9, the sodium-ion battery provided in Sample Example 10 has a mass ratio of sodium salt, non-aqueous organic solvent, and additive I of 12:79:9.

[0050] Sample Example 11 (A11) Compared to Sample Example 10, the sodium-ion battery provided in Sample Example 11 has a mass ratio of negative electrode active material, negative electrode conductive agent, negative electrode binder and additive II of 93.985:2.5:3.5:0.015.

[0051] Sample Example 12 (A12) Compared to Sample Example 11, the sodium-ion battery provided in Sample Example 12 has a mass ratio of sodium salt, non-aqueous organic solvent and additive I of 12:78.5:9.5.

[0052] Sample Example 13 (A13) Compared to Sample Example 6, the sodium-ion battery provided in Sample Example 13 uses 2,5-di-tert-butylpyrazine as additive II.

[0053] Sample Example 14 (A14) Compared to Sample Example 6, the sodium-ion battery provided in Sample Example 14 uses 2,5-diethoxypyrazine as additive II.

[0054] Sample Example 15 (A15) Compared to Sample Example 6, the sodium-ion battery provided in Sample Example 15 uses hexafluorocyclotriphosphazene (HFPN) as additive I.

[0055] Sample Example 16 (A16) Compared to Sample Example 6, the sodium-ion battery provided in Sample Example 16 uses pentafluoro(phenoxy)cyclotriphosphazene (FPPN) as additive I. Comparative Example 1 (D1) Compared to Sample Example 3, in the sodium-ion battery provided by Comparative Example 1, the mass ratio of negative electrode active material, negative electrode conductive agent, negative electrode binder and additive II is 93.5:2.5:3.5:0.5.

[0056] Comparative Example 2 (D2) Compared to Sample Example 3, in the sodium-ion battery provided by Comparative Example 2, the mass ratio of sodium salt, non-aqueous organic solvent and additive I is 12:76:12.

[0057] Comparative Example 3 (D3) Compared to Sample Example 1, the sodium-ion battery provided in Comparative Example 3 has a mass ratio of sodium salt, non-aqueous organic solvent and additive I of 12:86.8:1.2.

[0058] Comparative Example 4 (D4) Compared to Sample Example 12, in the sodium-ion battery provided by Comparative Example 4, the mass ratio of negative electrode active material, negative electrode conductive agent, negative electrode binder and additive II is 93.995:2.5:3.5:0.005.

[0059] Comparative Example 5 (D5) Compared to Sample Example 6, in the sodium-ion battery provided by Comparative Example 5, the mass ratio of sodium salt, non-aqueous organic solvent, and additive I is 12:88:0, and the mass ratio of negative electrode active material, negative electrode conductive agent, negative electrode binder, and additive II is 94:2.5:3.5:0. That is, in the sodium-ion battery provided by Comparative Example 5, additive II is not added to the negative electrode active material layer, and additive I is not added to the electrolyte.

[0060] Comparative Example 6 (D6) Compared to Sample Example 6, in the sodium-ion battery provided by Comparative Example 6, the mass ratio of sodium salt, non-aqueous organic solvent, and additive I is 12:88:0. That is, in the sodium-ion battery provided by Comparative Example 6, additive I is not added to the electrolyte.

[0061] Comparative Example 7 (D7) Compared to Sample Example 6, in the sodium-ion battery provided by Comparative Example 7, the mass ratio of negative electrode active material, negative electrode conductive agent, negative electrode binder, and additive II is 94:2.5:3.5:0. That is, in the sodium-ion battery provided by Comparative Example 7, additive II is not added to the negative electrode active material layer.

[0062] As can be seen from the above sample examples and comparative examples, the main variables among the examples are the types and mass contents of additive I and additive II. For a clear comparison, the main variables of each example are summarized in Table 1. Furthermore, according to the formula... The mass content of additive II x Converted to the mass content of additive II in the negative electrode active material layer and calculate The values ​​are listed in Table 1.

[0063] Table 1. Main variables and values ​​for each example

[0064] The sodium-ion batteries prepared in the above examples were subjected to impedance performance tests, rate cycle performance tests, high-temperature cycle performance tests, and first-cycle coulombic efficiency tests, respectively. The specific test methods are as follows: Impedance performance test: After the sodium-ion battery completes formation, its discharge DC internal resistance (DCIR) is measured.

[0065] Rate cycle performance test: The formed battery was placed in a 25℃ environment, first charged at 2C constant current to 3.9V, then charged at 3.9V constant voltage to the cutoff current of 0.05C, and then discharged at 2C constant current to 1.5V, cycled for 400 cycles, and the capacity retention rate was calculated according to the formula: Capacity retention rate (%) after 400 rate cycles = Discharge capacity in the 400th cycle / Discharge capacity in the 1st cycle × 100%.

[0066] High-temperature cycle performance test: Under 45℃ conditions, the formed battery is charged at a constant current of 0.7C to 3.9V, charged at a constant voltage of 3.9V to the cutoff current of 0.05C, and then discharged at a constant current of 1C to 1.5V, and cycled for 400 cycles. The capacity retention rate is calculated according to the formula: Capacity retention rate (%) after 400 cycles at 45℃ = Discharge capacity in the 400th cycle / Discharge capacity in the 1st cycle × 100%.

[0067] First-week coulombic efficiency test: The battery was charged at 0.1C constant current to 3.9V at 45℃, then charged at 3.9V constant voltage to form the battery until the cutoff current was 0.05C, and then discharged at 1C constant current to 1.5V. The initial discharge capacity was recorded, and the coulombic efficiency was calculated according to the formula: First-week coulombic efficiency (%) = initial discharge capacity / formation charge capacity × 100%.

[0068] The results of the above tests are shown in Table 2.

[0069] Table 2. Test results for each example

[0070] First, the difference between Samples 1 to 12 lies in the content and ratio of Additive I and Additive II. A comparison reveals that when the mass content of Additive I in the electrolyte... The mass content of additive II in the negative electrode active material layer is 0.1~10 wt%. Within the range of 1.5 to 3000 ppm, and Between 0.5 and 5, with a moderate increase in the value of a and a moderate decrease in the value of b (as in Samples 1 to 6), the DC internal resistance of the battery discharge gradually decreased from 71.8 mΩ to 68.4 mΩ, the capacity retention rate after 400 cycles at 45°C increased from 85.6% to 92.5%, the capacity retention rate after 400 cycles at high rate increased from 86.0% to 89.6%, and the first-cycle coulombic efficiency increased from 81.4% to 83.4%. This is attributed to the strong aromatic ring conjugation effect between additive II on the negative electrode surface and additive I with a higher content in the electrolyte, which enhances the interaction between the two, thereby improving the wetting effect of the electrolyte on the negative electrode surface, reducing polarization caused by incomplete wetting, and optimizing the composition of the solid electrolyte interphase (SEI) membrane. This results in a uniform distribution of highly stable sodium fluoride and low-resistance organic nitrogen-containing sodium salt in the SEI membrane, improving the mechanical strength of the SEI membrane and reducing its impedance, thus reducing the consumption of excess active sodium. Furthermore, it reduced the increased risk of sodium deposition caused by excessive additive II content, which in turn led to increased impedance. As the value of a continued to increase and the value of b continued to decrease (as in Samples 6 to 12), the battery discharge DC internal resistance gradually increased from 68.4 mΩ to 72.1 mΩ, the capacity retention rate after 400 cycles at 45°C decreased from 92.5% to 88.2%, the capacity retention rate after 400 cycles at high rate decreased from 89.6% to 86.6%, and the first-cycle coulombic efficiency decreased from 83.4% to 81.2%. This was attributed to the excessive content of additive I, which led to an excessively high content of sodium fluoride in the SEI film, and the excessively low content of additive II, which reduced the wetting effect of the electrolyte on the negative electrode surface and reduced the content of organic nitrogen-containing sodium salts in the SEI film, thus leading to increased impedance.

[0071] in, When the electrolyte is in the preferred range of 1 to 3 (such as Sample Examples 4 to 8), it is beneficial to further improve the wetting effect of the electrolyte on the negative electrode surface, reduce polarization, improve the first-cycle coulombic efficiency, and at the same time reduce impedance, thereby improving the capacity retention rate of sodium-ion batteries during rate cycles and high-temperature cycles. Specifically, the discharge DC internal resistance of sodium-ion batteries is reduced to below about 70 mΩ, the capacity retention rate after 400 rate cycles can reach more than 88%, and the capacity retention rate after 400 high-temperature cycles can reach more than 89.5%.

[0072] When the content and ratio of Additive I and Additive II exceed the above range, battery performance deteriorates. Specifically, compared with Sample Example 3, Comparative Example 1 had an excessive amount of Additive II, resulting in an excessively high content of pyrazine compounds in the negative electrode active material layer. This caused a steric hindrance effect, reducing the effective contact area between the electrolyte and the negative electrode active material, thereby increasing the battery's DC internal resistance during discharge, decreasing the cycle capacity retention rate, and reducing the first-cycle coulombic efficiency. Compared with Sample Example 3, Comparative Example 2 had an excessive amount of Additive I, resulting in an increase in the viscosity of the electrolyte system and excessive deposition of sodium fluoride on the surface of the negative electrode. This caused the battery's DC internal resistance during discharge to increase to 84.1 mΩ, and the capacity retention rate and first-cycle coulombic efficiency after 400 rate cycles decreased to 79.1% and 78.4%, respectively. The capacity retention rate after 400 cycles at 45°C decreased to 77.2%. Deviations from the 0.5~5 range were observed. For example, in Comparative Example 3, insufficient Additive I led to a relative excess of Additive II on the negative electrode surface, causing a steric hindrance effect and consequently increasing the battery's DC internal resistance to 82.7 mΩ. In Comparative Example 4, insufficient Additive II caused excessive inorganicization of the solid electrolyte interface film, i.e., excessive deposition of sodium fluoride on the negative electrode surface, resulting in a decrease in the first-week coulombic efficiency to 74.9%. This also confirms... a and b The correlation is crucial for SEI membrane regulation.

[0073] Secondly, the difference between Sample 6 and Samples 13 to 16 lies in the different molecular structures of Additive I and Additive II. A comparison shows that when maintaining... , , Despite the differences in the specific substances used in additive II or additive I, the fluctuations in the battery's DC internal resistance during discharge, cycle capacity retention, and first-cycle coulombic efficiency were all relatively small, indicating that both additive I with different fluorinated triphosphazene compounds and additive II with different pyrazine compounds can improve battery performance.

[0074] Furthermore, the difference between Sample 6 and Comparative Examples 5 to 7 lies in the presence or absence of Additive I and Additive II. Comparison reveals that when Additive I is absent (Comparative Example 6), the solid electrolyte interface membrane lacks the sodium fluoride produced by the decomposition of Additive I, resulting in insufficient mechanical strength, easy rupture, and continuous consumption of active sodium ions. This leads to an increase in its discharge DC internal resistance to 88.1 mΩ, and both the rate cycle capacity retention and high-temperature cycle capacity retention are below 74%, with a first-cycle coulombic efficiency of only 72.4%. When Additive II is absent (Comparative Example 7), due to the lack of pyrazine compounds, the cyclic structure of Additive I cannot produce an aromatic ring conjugation effect, while the added... The high viscosity of Additive I and the high sodium fluoride content in the SEI film resulted in insufficient wettability of the electrolyte to the negative electrode, an increased sodium ion migration barrier, and increased battery polarization. Consequently, the battery's DC discharge internal resistance increased to 88.6 mΩ, and the first-cycle coulombic efficiency decreased to 73.1%. When both Additive I and Additive II were absent (Comparative Example 5), the solid electrolyte interfacial film was mainly formed by the decomposition of non-aqueous organic solvents, resulting in a loose, porous, and easily ruptured interface. This led to an unstable interface, with a DC discharge internal resistance as high as 89.3 mΩ, a capacity retention rate of only 72.1% after 400 cycles at 45°C, a capacity retention rate of only 70.8% after 400 rate cycles, and a first-cycle coulombic efficiency as low as 71.5%.

[0075] In summary, this invention enhances the interaction between the electrolyte and the negative electrode surface through the strong aromatic ring conjugation effect of the cyclic structure of additive I and the pyrazine ring of additive II, thereby improving the wettability of the electrolyte. Simultaneously, additive II contains nitrogen, which can generate a nitrogen-containing organic solid electrolyte interfacial film. This solid electrolyte interfacial film contains organic nitrogen-containing sodium salts to improve the sodium fluoride interface, enhance interfacial mechanical strength, reduce interfacial impedance, and optimize interfacial stability, thus improving the cycle life of the sodium-ion battery. This has been fully verified through multi-dimensional performance testing and structural variation comparison analysis of the sample examples and comparative examples.

[0076] The above description is merely an embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principle of the present invention should be included within the scope of the claims of the present invention.

Claims

1. A sodium-ion battery, characterized in that, The device includes a positive electrode, a negative electrode, and an electrolyte; wherein the electrolyte includes a sodium salt, a non-aqueous organic solvent, and additive I, wherein additive I is a fluorinated cyclotriphosphazene compound; the negative electrode includes a negative current collector and a negative active material layer, wherein the negative active material layer is coated on at least one surface of the negative current collector, and the negative active material layer includes a negative active material and additive II, wherein additive II is a pyrazine compound.

2. The sodium-ion battery according to claim 1, characterized in that, The mass content of additive I in the electrolyte is: The mass content of additive II in the negative electrode active material layer is: , .

3. The sodium-ion battery according to claim 2, characterized in that, The mass content of additive I in the electrolyte Mass content of Additive II in the negative electrode active material layer The following relationship must be satisfied: .

4. The sodium-ion battery according to claim 3, characterized in that, The mass content of additive I in the electrolyte Mass content of Additive II in the negative electrode active material layer The following relationship must be satisfied: .

5. The sodium-ion battery according to claim 1, characterized in that, The additive I is selected from one or more of ethoxy (pentafluoro)cyclotriphosphazene, hexafluorocyclotriphosphazene, and pentafluoro (phenoxy)cyclotriphosphazene.

6. The sodium-ion battery according to claim 2, characterized in that, The mass content of additive I in the electrolyte The percentages are 2wt%, 3wt%, 4.5wt%, 7wt%, 8wt%, 9wt%, or 9.5wt%.

7. The sodium-ion battery according to claim 1, characterized in that, Additive II is selected from pyrazine compounds containing hydrocarbon groups.

8. The sodium-ion battery according to claim 7, characterized in that, Additive II is selected from one or more of 2,5-dibutylpyrazine, 2,5-di-tert-butylpyrazine and 2,5-diethoxypyrazine.

9. The sodium-ion battery according to claim 2, characterized in that, The mass content of additive II in the negative electrode active material layer The values ​​are 150ppm, 300ppm, 700ppm, 1500ppm, 2100ppm, or 2500ppm.

10. The sodium-ion battery according to claim 1, characterized in that, The negative electrode active material is selected from one or more of hard carbon, soft carbon, sodium titanate, metallic sodium, and metals that can form alloys with sodium.

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