Sodium-ion battery electrolyte resistant to floating charge and application thereof

Abstract

The application discloses a sodium-ion battery electrolyte resistant to floating charging and application thereof, which comprises a sodium salt, a first additive, a second additive and an organic solvent; wherein the first additive is a compound containing an SO3 ‑ group and a hydrophobic skeleton, and the second additive is a multifunctional polymer containing a polyoxyethylene ether chain segment. The application adds a two-component additive. Under specific formation conditions of the battery, the first additive is preferentially adsorbed and reduced on the negative electrode surface to occupy active sites, and provides a nucleation basis for the construction of a subsequent SEI film. The second additive undergoes a cross-linking reaction on the negative electrode surface through a mechanism such as free radical polymerization, polycondensation or ring-opening polymerization, to form a flexible organic SEI film with a three-dimensional network structure, which realizes complete coating of the hard carbon negative electrode, thereby effectively inhibiting side reactions under the floating charging condition, and improving the safety and cycle life of the battery.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and more specifically to a sodium-ion battery electrolyte that is resistant to float charging and its application. Background Technology

[0002] Sodium-ion batteries, due to the abundance and low cost of sodium resources and their similarity in working principle to lithium-ion batteries, show broad application prospects in large-scale energy storage. Currently, sodium-ion batteries mostly use polyanionic materials for the positive electrode and hard carbon materials for the negative electrode.

[0003] In specific applications such as uninterruptible power supplies (UPS), batteries need to be in a float charge state for extended periods (i.e., maintained at a constant voltage for a long time). Under this condition, side reactions continuously occur on the negative electrode side of traditional sodium-ion batteries, where the electrolyte is reduced and decomposed, generating gases such as H2 and CH4. This not only increases the internal pressure of the battery, posing safety risks, but also causes loss of active materials and degradation of battery performance, severely limiting the application of sodium-ion batteries in fields such as UPS.

[0004] To improve battery performance, various electrolyte improvement schemes have been proposed in the prior art. For example, patent CN113299989A discloses an electrolyte containing silanes or sulfur-containing compounds with specific structures as additives, aiming to form a uniform and dense SEI film to improve the battery's cycle life and high-temperature storage performance. Another example is Chinese patent application CN115911544A, which discloses an electrolyte containing fluorooxalate phosphate and sodium saccharin, using their similar reduction potentials to jointly construct the SEI film, overcoming problems such as acid production from the decomposition of single additives and high impedance.

[0005] However, the aforementioned existing technologies primarily focus on improving the battery's performance during normal cycles or high-temperature storage, and do not offer effective solutions to the persistent side reactions at the negative electrode under the special operating conditions of UPS float charging. Furthermore, film-forming additives commonly used in sodium-ion batteries, such as FEC (fluoroethylene carbonate), form SEI films primarily composed of inorganic substances like NaF. These inorganic SEI films typically grow via "point-like nucleation," which makes it difficult to form a complete and uniform coating layer for hard carbon negative electrode materials, which have a structure characterized by "short-range order and long-range disorder" and numerous surface defects. Therefore, under harsh long-term float charging conditions, this incomplete SEI film cannot effectively isolate the electrolyte from the negative electrode, leading to continuous side reactions and rapid degradation of battery performance.

[0006] Therefore, how to suppress the negative electrode side reaction during float charging and improve the battery's float charging tolerance has become an urgent technical problem to be solved. Summary of the Invention

[0007] This invention addresses the shortcomings of existing technologies by providing a float-charge resistant sodium-ion battery electrolyte and its application. It involves adding a two-component additive. Under specific battery formation conditions, the first additive preferentially adsorbs and reduces to occupy active sites on the negative electrode surface, providing a nucleation basis for the subsequent construction of the SEI film. The second additive undergoes a cross-linking reaction on the negative electrode surface through mechanisms such as free radical polymerization, condensation polymerization, or ring-opening polymerization, forming a flexible organic SEI film with a three-dimensional network structure. This achieves complete coating of the hard carbon negative electrode, effectively suppressing side reactions under float-charge conditions and improving battery safety and cycle life.

[0008] To solve the above-mentioned technical problems, the first aspect of the present invention provides a sodium-ion battery electrolyte resistant to float charging, comprising sodium salt, a first additive, a second additive, and an organic solvent;

[0009] The first additive contains SO3 - The compound has functional groups and a hydrophobic framework, wherein the second additive is a multifunctional polymer containing polyoxyethylene ether segments.

[0010] Furthermore, the first additive is selected from one or more of sodium nonylphenol polyoxyethylene ether sulfate (NPES), sodium dinonylnaphthalene sulfonate, and sodium alkylbenzene sulfonate.

[0011] Furthermore, the second additive is selected from one or more of ethoxylated alkylphenol formaldehyde resin, polyoxyethylene ether sulfate (APEO), polyethylene glycol diacrylate (PEGDA), polyethylene glycol dialkyl acrylate (PEGDAE), polyethylene glycol diglycidyl ether (PEGDE), and polyethylene glycol diamine (PEGDAm); the number average molecular weight of the second additive is ≥500.

[0012] This invention specifically adds a two-component additive to the electrolyte:

[0013] The first additive, as a polar anionic surfactant, contains highly polar SO3 in its molecular structure. - This gives it strong surface activity and adsorption capacity, enabling it to disperse uniformly in the electrolyte and preferentially adsorb onto the negative electrode surface during specific low-current formation processes. During the initial charging process, these molecules preferentially undergo reductive decomposition over carbonate solvents, and the resulting products rapidly occupy highly active sites (such as defects and edge structures) on the hard carbon surface, thus physically isolating solvent molecules from direct contact with the negative electrode and inhibiting continuous solvent reductive decomposition and gas generation reactions at the source. Furthermore, the first additives are all in sodium salt form, and the Na+ released after dissociation in the electrolyte... + It can increase the concentration of freely moving sodium ions in the system to a certain extent, which plays an auxiliary role in improving ionic conductivity and forms a synergistic ion supply system with the main sodium salt.

[0014] The second additive has a large average molecular weight, its molecular chains possess good flexibility, and it contains multiple active reaction sites (one or more of hydroxymethyl, sulfate, acrylate, alkenyl, epoxy, and amino groups). During the main formation process, these molecules undergo cross-linking reactions on the negative electrode surface, forming a flexible organic SEI film. Because hard carbon materials have a short-range ordered and long-range disordered structure with numerous surface defects, the inorganic SEI components formed by traditional additives (such as NaF and Na2O) are usually point-like nuclei, making it difficult to form a complete and uniform coating layer. However, the organic SEI film with a three-dimensional network structure formed by the second additive through cross-linking reactions via free radical polymerization, condensation polymerization, or ring-opening polymerization provides more complete and comprehensive coverage, offering flexible and dense protection for the hard carbon negative electrode and effectively isolating the electrolyte from direct contact with the negative electrode.

[0015] Furthermore, the first additive has a mass percentage of 0.1-10 wt% in the electrolyte, and the second additive has a mass percentage of 0.01-0.2 wt% in the electrolyte.

[0016] Furthermore, the sodium salt is selected from one or more of sodium hexafluorophosphate, sodium perchlorate, and sodium tetrafluoroborate.

[0017] Furthermore, the concentration of sodium salt in the electrolyte is 0.5-2M.

[0018] Furthermore, the organic solvent is selected from one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0019] A second aspect of the present invention provides a sodium-ion battery, comprising the electrolyte, positive electrode and hard carbon negative electrode described in the first aspect.

[0020] Furthermore, the formation method of the sodium-ion battery includes the following steps:

[0021] S1. Charge at a constant current of 0.01C ~ 0.1C until the voltage reaches 0.5V ~ 1.5V.

[0022] S2. Charge at a constant current of 0.05C ~ 0.2C until the voltage reaches 2.5V ~ 3.2V.

[0023] Furthermore, S2 also includes:

[0024] (1) Place the battery in an environment of 35℃~55℃ for 12 h~48 h to age, and seal it after degassing;

[0025] (2) Perform 1 to 3 charge-discharge cycles at a current of 0.2C to 0.5C to complete battery activation.

[0026] Furthermore, before S1, the process also includes: after injecting the electrolyte into the battery, allowing it to stand for 12 h to 48 h at 25℃ to 45℃.

[0027] The beneficial effects of this invention are:

[0028] This invention adds a two-component additive to the electrolyte. Under low-current formation conditions, the first additive preferentially adsorbs and reduces to occupy active sites on the negative electrode surface, thereby physically isolating solvent molecules from direct contact with the negative electrode. This inhibits the continuous reduction and decomposition of the solvent and gas generation reaction from the source, and provides a nucleation basis for the subsequent construction of the SEI film. The second additive undergoes a cross-linking reaction on the negative electrode surface through mechanisms such as free radical polymerization, condensation polymerization or ring-opening polymerization during the main formation stage, forming a flexible organic SEI film with a three-dimensional network structure, which completely covers the hard carbon negative electrode. This effectively suppresses side reactions under float charging conditions and improves the safety and cycle life of the battery.

[0029] This invention utilizes the synergistic effect of the first and second additives to form a composite SEI film with a "dense bottom layer and flexible top layer" on the surface of the hard carbon anode, fundamentally suppressing gas generation and side reactions during the float charging process. Attached Figure Description

[0030] To more clearly illustrate the technical solution of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 These are line graphs showing the capacity retention rate of batteries after float charging, representing embodiments and comparative examples of the present invention. Detailed Implementation

[0032] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. 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.

[0033] Example 1

[0034] This embodiment relates to a method for preparing a sodium-ion battery, comprising the following steps:

[0035] (1) Electrolyte preparation: In a glove box filled with argon (water and oxygen content are both below 0.1 ppm), NaPF6 (sodium hexafluorophosphate) was dissolved at a concentration of 1M in a mixed solvent of EC (ethylene carbonate):DEC (diethyl carbonate):EMC (methyl ethyl carbonate) = 1:1:1 (volume ratio). After stirring evenly, 3% by mass of sodium nonylphenol polyoxyethylene ether sulfate (NPES, molecular weight 540) was added as the first additive, and 0.1% by mass of ethoxylated alkylphenol formaldehyde resin (number average molecular weight 10000) and 0.05% by mass of PEGDA (number average molecular weight 700) were added as the second additive. The mixture was stirred evenly to obtain the electrolyte.

[0036] (2) Battery Assembly: Polyanionic material (sodium iron pyrophosphate) was used as the main positive electrode material, and hard carbon was used as the main negative electrode material. The main material, conductive agent 1 (acetylene black), conductive agent 2 (carbon nanotubes), and binder were added to the slurry mixer in a certain order according to a mass ratio of 95.0:2.0:1.0:2.0. An appropriate amount of NMP (N-methylpyrrolidone) was added as a solvent, and the solid content was controlled at 60wt%. The mixture was dispersed at high speed for 200 min, and the vacuum degree was controlled at -0.085MPa during the process. The uniformly mixed slurry was transferred to the coating machine, and the coating machine oven was turned on in advance, and the oven temperature was controlled at 100℃. The surface density of the double-sided dry film coated on the positive electrode was 12mg / cm³. 2 The density of the double-sided dry film coated on the negative electrode is 10 mg / cm³. 2 The coated electrode rolls were rolled at 25°C and 5.0% RH. The corresponding thicknesses were 100 μm for the positive electrode and 105 μm for the negative electrode. The electrode sheets were then die-cut and grouped, and the positive and negative electrodes were stacked in a Z-shaped manner to form a core package for prismatic battery. The prepared electrolyte was then injected, and the battery entered the formation stage.

[0037] (3) Transformation stage:

[0038] After the electrolyte is injected into the assembled prismatic battery, it is left to stand in a 35°C constant temperature chamber for 24 hours to allow the electrolyte to fully wet the positive and negative electrode plates and the separator.

[0039] The battery is pre-charged at a constant current of 0.05C until the voltage reaches 1.0 V. During this step, an initial SEI film is formed on the surface of the hard carbon anode under the preferential adsorption and reductive decomposition of the first additive.

[0040] Continue charging at a constant current of 0.1C until the voltage reaches 3.0 V. In this step, the second additive undergoes a cross-linking reaction, further forming a flexible organic SEI network on the initial SEI film layer, together constructing a composite SEI film with a "dense bottom layer + flexible top layer".

[0041] The battery was aged in a constant temperature environment of 45℃ for 24 hours to promote further stabilization and reorganization of the SEI film, making the film structure more dense and uniform.

[0042] Under inert gas protection, the battery is degassed under vacuum to remove the small amount of gas generated during the formation process, and then the liquid injection port is finally sealed.

[0043] The battery was activated and its capacity calibrated by performing three complete charge-discharge cycles at a current of 0.2C (charging cut-off voltage 3.8V, discharging cut-off voltage 1.5V).

[0044] Example 2

[0045] The difference between this embodiment and Example 1 is that the additives in the electrolyte are replaced with: sodium dinonylnaphthalenesulfonate (molecular weight 483) 2% + APEO (number average molecular weight 1000) 0.15% + PEGDAE (number average molecular weight 700) 0.05%, while other steps and parameters remain unchanged.

[0046] Example 3

[0047] The difference between this embodiment and Example 1 is that the additives in the electrolyte are replaced with: sodium alkylbenzene sulfonate (molecular weight 348) 1.5% + PEGDE (number average molecular weight 1200) 0.1% + PEGDAm (number average molecular weight 2000) 0.03%, while other steps and parameters remain unchanged.

[0048] Comparative Example 1

[0049] The difference between this comparative example and Example 1 is that the second additive is omitted, while the other steps and parameters remain unchanged.

[0050] Comparative Example 2

[0051] The difference between this comparative example and Example 1 is that the first additive is omitted, while other steps and parameters remain unchanged.

[0052] Comparative Example 3

[0053] The difference between this comparative example and Example 1 is that the first and second additives are omitted, while other steps and parameters remain unchanged.

[0054] Comparative Example 4

[0055] The difference between this comparative example and Example 1 is that the first and second additives are omitted, and the conventional additive fluoroethylene carbonate (FEC) is added at a mass ratio of 3 wt%, while other steps and parameters remain unchanged.

[0056] Test case

[0057] The activated batteries from both the example and comparative examples were placed in a constant temperature environment of 55°C and subjected to float charging tests at 3.40V. Capacity calibration was performed at a current of 0.5C at weeks 1, 2, 3, 4, 8, and 12 after the start of float charging, and the capacity retention rate was recorded. Simultaneously, battery swelling was observed. The results are as follows: Figure 1 As shown in the figure, the results for 12 weeks are shown in Table 1.

[0058] Table 1

[0059]

[0060] As shown in Table 1, the batteries obtained in Examples 1-3 exhibit excellent stability under float charging conditions.

[0061] Example 1: The NPES electrolyte contains both sulfonate groups and EO (polyoxyethylene ether) segments, possessing the dual functions of preferential film formation and ion conduction. The sulfonate groups in the molecule allow it to preferentially adsorb onto the negative electrode surface, rapidly occupying highly active sites on the hard carbon surface during the low-current formation stage, physically isolating solvent molecules, and inhibiting solvent reduction decomposition and gas generation reactions from the source. The EO segments can complex sodium ions, promoting the dissociation of NaPF6 and increasing the free movement of Na+. + Concentration; simultaneously, the EO chain segment itself has the ability to conduct sodium ions, thus becoming Na. + It provides additional hopping sites and reduces interfacial impedance. Alkylphenol formaldehyde resin, as a high molecular weight polymer, has a flexible molecular chain and contains multiple hydroxymethyl active sites (12%-16%). During the formation process, it undergoes a cross-linking reaction to form a flexible organic SEI film framework. The acrylate double bonds at both ends of PEGDA can participate in free radical polymerization and interpenetrate with the resin cross-linking network, further enhancing the integrity and flexibility of the SEI film. The EO segments in the middle also contribute to ion conduction. The alkylphenol formaldehyde resin + PEGDA dual polymer system forms an interpenetrating network structure, resulting in the best mechanical strength and flexibility of the SEI film. In this embodiment, the initial film-forming layer formed by the first type of additive provides a good nucleation substrate for the cross-linking reaction of the second type of additive. Together, they construct a composite SEI film with a "dense bottom layer + flexible top layer," achieving complete coating of the hard carbon anode and exhibiting optimal stability.

[0062] In Example 2, sodium dinonylnaphthalene sulfonate in the electrolyte does not contain EO segments, resulting in a weaker contribution to ion conduction. APEO and PEGDAE are both EO polymers, exhibiting good flexibility but with a slightly lower crosslinking density than the resin system in Example 1, thus slightly reducing the capacity retention rate. In Example 3, sodium alkylbenzene sulfonate has a smaller molecular weight, leading to slightly less dense film formation. PEGDE and PEGDAm undergo ring-opening crosslinking via epoxy-amine groups, resulting in a SEI film with good chemical stability but slightly lower flexibility, thus slightly reducing the capacity retention rate.

[0063] The electrolyte in Comparative Example 1 contains a first additive, which can preferentially form a film and occupy the active sites, but lacks cross-linking and coating of high molecular weight polymers. The resulting SEI film is mainly composed of inorganic components and grows in a "point-like nucleation" manner, making it difficult to completely cover the defects on the hard carbon surface. Solvent still penetrates under long-term float charging.

[0064] In Comparative Example 2, the electrolyte lacked the preferential film formation and site occupation of the first additive, the second type of polymer crosslinking reaction lacked a good nucleation substrate, the crosslinking network had insufficient bonding force with the negative electrode surface, and the coating uniformity was limited.

[0065] In Comparative Example 3, solvent molecules in the electrolyte directly contact the negative electrode surface, continuously undergoing reduction and decomposition reactions to generate gases such as H2 and CH4, leading to severe battery swelling and rapid capacity decay.

[0066] In Comparative Example 4, the FEC forms an SEI film mainly composed of inorganic components (such as NaF), which grows on the hard carbon surface through a "point-like nucleation" method, failing to completely cover its structural defects. Under long-term float charging conditions, solvent decomposition and side reactions will continue to occur at the incomplete SEI film, producing gases such as H2 and CH4, leading to rapid capacity decay and swelling of the battery.

[0067] In summary, after 12 weeks of high-temperature float charging testing at 55°C, the battery in Comparative Example 3 (without additives) exhibited severe swelling, with a capacity retention rate below 60%; the batteries in Comparative Examples 1 and 2 showed slight swelling, with a capacity retention rate of approximately 75-80%; while the batteries in Examples 1-3 showed no significant swelling, and their capacity retention rate remained above 90%. This demonstrates that the present invention, through the synergistic effect of the two additives, significantly improves the stability of sodium-ion batteries under high-temperature float charging conditions, effectively suppresses negative electrode side reactions and gas generation, and extends battery life.

[0068] The present invention has been described in detail above with reference to specific embodiments and exemplary examples; however, these descriptions should not be construed as limiting the present invention. Those skilled in the art will understand that various equivalent substitutions, modifications, or improvements can be made to the technical solutions and embodiments of the present invention without departing from the spirit and scope of the invention, and all such modifications and improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A sodium-ion battery electrolyte resistant to float charging, characterized in that, Includes sodium salt, first additive, second additive, and organic solvent; The first additive contains SO3 - The compound has functional groups and a hydrophobic framework, wherein the second additive is a multifunctional polymer containing polyoxyethylene ether segments.

2. The sodium-ion battery electrolyte resistant to float charging as described in claim 1, characterized in that, The first additive is selected from one or more of sodium nonylphenol polyoxyethylene ether sulfate, sodium dinonylnaphthalene sulfonate, and sodium alkylbenzene sulfonate.

3. The sodium-ion battery electrolyte resistant to float charging as described in claim 1, characterized in that, The second additive is selected from one or more of ethoxylated alkylphenol formaldehyde resin, polyoxyethylene ether sulfate, polyethylene glycol diacrylate, polyethylene glycol dialkyl acrylate, polyethylene glycol diglycidyl ether, and polyethylene glycol diamine; the number average molecular weight of the second additive is ≥500.

4. The sodium-ion battery electrolyte resistant to float charging as described in claim 1, characterized in that, The first additive has a mass percentage of 0.1-10 wt% in the electrolyte, and the second additive has a mass percentage of 0.01-0.2 wt% in the electrolyte.

5. The sodium-ion battery electrolyte resistant to float charging as described in claim 1, characterized in that, The sodium salt is selected from one or more of sodium hexafluorophosphate, sodium perchlorate, and sodium tetrafluoroborate.

6. The sodium-ion battery electrolyte resistant to float charging as described in claim 1, characterized in that, The concentration of sodium salt in the electrolyte is 0.5-2M.

7. The sodium-ion battery electrolyte resistant to float charging as described in claim 1, characterized in that, The organic solvent is selected from one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

8. A sodium-ion battery, characterized in that, Includes the electrolyte, positive electrode, and hard carbon negative electrode as described in any one of claims 1-7.

9. The sodium-ion battery as described in claim 8, characterized in that, The formation method of the sodium-ion battery includes the following steps: S1. Charge at a constant current of 0.01C ~ 0.1C until the voltage reaches 0.5V ~ 1.5V. S2. Charge at a constant current of 0.05C ~ 0.2C until the voltage reaches 2.5V ~ 3.2V.

10. The sodium-ion battery as described in claim 9, characterized in that, S2 also includes: (1) Place the battery in an environment of 35℃~55℃ for 12 h~48 h to age, and seal it after degassing; (2) Perform 1 to 3 charge-discharge cycles at a current of 0.2C to 0.5C to complete battery activation.

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