Non-aqueous electrolyte for sodium-ion battery and sodium-ion battery

Abstract

To overcome the technical problem that the solubility of sodium monofluorophosphate in the existing non-aqueous electrolyte for sodium ion batteries is low, resulting in the reduction of the initial efficiency, rate capability and cycle performance of the battery, the application provides a non-aqueous electrolyte for sodium ion batteries and a sodium ion battery, the non-aqueous electrolyte comprising a non-aqueous organic solvent, a sodium salt and an additive, the non-aqueous organic solvent comprising propylene carbonate, the additive comprising sodium monofluorophosphate and fluoroethylene carbonate, the non-aqueous electrolyte satisfying the following conditions: 0.6 <= a x d <= 15, and d = b / c; wherein 0.1 <= a <= 3, 5 <= b <= 50, 1 <= c <= 5, and 3 <= d <= 10; the non-aqueous electrolyte provided by the application has the synergistic effect of sodium monofluorophosphate, propylene carbonate and fluoroethylene carbonate, forming a stable interface film at the positive and negative electrode interface, improving the transmission rate of sodium ions on the interface film, inhibiting the decomposition of the non-aqueous electrolyte, reducing the interface film impedance and growth rate, ensuring the normal play of the initial efficiency of the battery, improving the cycle performance, low-temperature performance and rate capability of the battery.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, specifically relating to a non-aqueous electrolyte for sodium-ion batteries and a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries are similar in principle and structure to lithium-ion batteries. Compared with lithium batteries, sodium-ion batteries have more abundant resources, lower cost and less fluctuation, and have the potential to replace lithium batteries due to their wide temperature range and high safety performance. With the continuous advancement of sodium-ion battery technology, sodium-ion batteries will occupy an important position in my country's energy system, especially in the field of energy storage, where they have broad growth potential. Therefore, the development of high-performance, safe and stable sodium-ion batteries is the decisive factor in determining whether they can be industrialized.

[0003] Existing sodium-ion batteries typically use non-aqueous electrolytes containing at least one of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium perchlorate, or other organic sodium salts, with organic solvents such as propylene carbonate as the primary solvent. Currently, sodium monofluorophosphate is rarely used as an additive in sodium-ion batteries. This is because, compared to lithium monofluorophosphate's use as a secondary electrolyte salt or additive in lithium-ion batteries, sodium monofluorophosphate is extremely difficult to dissolve as an additive in sodium-ion batteries. Insufficient sodium monofluorophosphate content in non-aqueous electrolytes affects the formation of the electrode interface film, reducing the battery's cycle performance. While using large amounts of propylene carbonate (PC) can improve the solubility of sodium monofluorophosphate, excessive amounts of propylene carbonate can lead to a decrease in the battery's initial efficiency and rate performance.

[0004] Therefore, how to effectively solve the problem of low solubility of sodium monofluorophosphate in non-aqueous electrolytes for sodium-ion batteries, which leads to reduced initial efficiency, rate capability, and cycle performance of sodium-ion batteries, is an urgent research topic. Summary of the Invention

[0005] To address the technical problem of low solubility of sodium monofluorophosphate in non-aqueous electrolytes for sodium-ion batteries, which leads to reduced initial efficiency, rate capability, and cycle performance, this application provides a non-aqueous electrolyte for sodium-ion batteries and a sodium-ion battery. The non-aqueous electrolyte uses sodium monofluorophosphate as an additive, in combination with propylene carbonate and fluoroethylene carbonate. The added fluoroethylene carbonate reduces the amount of propylene carbonate used while effectively increasing the solubility of sodium monofluorophosphate, thereby improving the battery's initial efficiency, rate capability, and cycle performance.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows:

[0007] On one hand, this application provides a non-aqueous electrolyte for sodium-ion batteries, the non-aqueous electrolyte comprising a non-aqueous organic solvent, a sodium salt and additives, the non-aqueous organic solvent comprising propylene carbonate, and the additives comprising sodium monofluorophosphate and fluoroethylene carbonate;

[0008] The non-aqueous electrolyte meets the following conditions:

[0009] 0.6 ≤ a × d ≤ 15, and d = b / c;

[0010] Where 0.1≤a≤3, 5≤b≤50, 1≤c≤5, 3≤d≤10;

[0011] a represents the mass percentage of sodium monofluorophosphate in the non-aqueous electrolyte, expressed in %.

[0012] b represents the mass percentage of propylene carbonate in the non-aqueous electrolyte, expressed as %.

[0013] c represents the mass percentage of fluoroethylene carbonate in the non-aqueous electrolyte, expressed as %.

[0014] Preferably, the ratio d of the mass percentage of propylene carbonate to the mass percentage of fluoroethylene carbonate is in the range of 3 ≤ d ≤ 10.

[0015] The non-aqueous electrolyte meets the following conditions:

[0016] 0.9≤a×d≤12.

[0017] Preferably, the mass percentage (a%) of the sodium monofluorophosphate in the non-aqueous electrolyte ranges from 0.2% to 1.5%.

[0018] Preferably, the mass percentage b% of the propylene carbonate in the non-aqueous electrolyte ranges from 7% to 30%.

[0019] Preferably, the mass percentage (c%) of the fluoroethylene carbonate in the non-aqueous electrolyte ranges from 1.5% to 3%.

[0020] Preferably, the sodium salt comprises at least one of sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium trifluoromethanesulfonylimide, sodium trifluoromethanesulfonate, sodium tetrafluoroborate, sodium perchlorate, sodium bis(oxalato)borate, sodium difluorooxalatoborate, sodium hexafluoroarsenate, sodium trifluoroacetate, sodium tetraphenylborate, and sodium bis(trifluoromethanesulfonyl)imide.

[0021] Preferably, the sodium salt content is 12% to 14% by mass, based on 100% of the non-aqueous electrolyte.

[0022] Preferably, the non-aqueous organic solvent further includes at least one of cyclic or chain carbonates with 3 to 5 carbon atoms, carboxylic acid esters with 2 to 6 carbon atoms, and cyclic or chain ethers with 4 to 10 carbon atoms.

[0023] Preferably, the additive further includes auxiliary additives, which include at least one of vinylene carbonate, ethylene carbonate, ethylene sulfate, 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, 1,4-butanesulfonate lactone, and difluoroethylene carbonate.

[0024] The non-aqueous electrolyte accounts for 100% of the total mass, and the auxiliary additives account for 0.01% to 5% of the total mass.

[0025] On the other hand, this application provides a sodium-ion battery, including a positive electrode, a negative electrode, and the aforementioned non-aqueous electrolyte for sodium-ion batteries.

[0026] Preferably, the positive electrode includes a positive electrode active material, which includes at least one of layered transition metal oxides, Prussian compounds, phosphate compounds, and sulfate compounds; the negative electrode includes a negative electrode active material, which includes at least one of hard carbon, soft carbon, carbon nanotubes, graphite, graphene, expanded graphite, or mesophase carbon microspheres.

[0027] Preferably, the layered transition metal oxide comprises a compound represented by Formula I:

[0028] Na x M y O z Formula I

[0029] Wherein, 0 < x ≤ 1, 0 < y ≤ 1, 1 < z ≤ 2, and M is selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V;

[0030] The Prussian-type compounds include those represented by Formula II:

[0031] Na x′ L y′ [L′(CN)6] y′ ·z′H2O Formula II

[0032] Wherein, 0 < x′ ≤ 2, 0 < y′ ≤ 1, 0 < z′ ≤ 20, and L and L′ are each selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V;

[0033] The phosphate compound includes at least one of the compounds represented by Formula III or Formula IV:

[0034] Na3(M′O1-q PO4)2F 1+2q Formula III

[0035] Where 0≤q≤1, M′ is selected from at least one of Al, V, Ge, Fe, and Ga;

[0036] Na2EPO4F Formula IV

[0037] Wherein, E is selected from at least one of Fe and Mn;

[0038] The sulfate compounds include at least one of the compounds represented by Formula V:

[0039] Na2Y(SO4)2·2H2O (Formula V)

[0040] Wherein, Y is selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V.

[0041] Beneficial effects:

[0042] The non-aqueous electrolyte for sodium-ion batteries provided in this application uses sodium monofluorophosphate (Na2PO3F) as an additive, which can form a film on the positive and negative electrode surfaces. This suppresses the side reactions caused by the decomposition of the non-aqueous electrolyte due to contact between the non-aqueous electrolyte and the positive and negative electrode active materials, significantly improving the stability of the electrodes and the non-aqueous electrolyte, reducing irreversible loss of active sodium, enhancing cycle reversibility, and promoting cycle stability. More importantly, the inventors have discovered through extensive research that when the ratio d = b / c of ​​the mass percentage of sodium monofluorophosphate (a%), propylene carbonate (c%), and fluoroethylene carbonate (b%) in the non-aqueous electrolyte satisfies the conditions 0.6 ≤ a × d ≤ 15 and 3 ≤ d ≤ 10, the sodium monofluorophosphate, propylene carbonate, and fluoroethylene carbonate can be fully utilized. The synergistic effect of these three components is as follows: the addition of propylene carbonate increases the solubility of sodium monofluorophosphate in the electrolyte, significantly improving its solubility and enabling it to effectively function as an additive and participate in film formation; the addition of fluoroethylene carbonate reduces the amount of propylene carbonate used, ensuring the solubility of sodium monofluorophosphate while avoiding the decrease in initial efficiency and rate performance caused by excessive use of propylene carbonate solvent. It also effectively improves the conductivity of the electrolyte, ensuring that the viscosity of the battery meets the requirements at low temperatures and that its low-temperature performance is not degraded. The combined effect of these three components improves the film quality of the positive and negative electrodes, increases the content of fluorinated compounds in the interfacial film, increases the transport rate of sodium ions on the interfacial film, reduces the impedance and growth rate of the interfacial film, and effectively improves the cycle stability and rate performance of the battery. Detailed Implementation

[0043] To make the technical problems solved, the technical solutions, and the beneficial effects of the present invention clearer, the present invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present invention and are not intended to limit the present invention.

[0044] On one hand, this application provides a non-aqueous electrolyte for sodium-ion batteries, the non-aqueous electrolyte comprising a non-aqueous organic solvent, a sodium salt, and additives, wherein the non-aqueous organic solvent comprises propylene carbonate, and the additives comprise sodium monofluorophosphate and fluoroethylene carbonate.

[0045] The non-aqueous electrolyte meets the following conditions:

[0046] 0.6 ≤ a × d ≤ 15, and d = b / c;

[0047] Where 0.1≤a≤3, 5≤b≤50, 1≤c≤5, 3≤d≤10;

[0048] a represents the mass percentage of sodium monofluorophosphate in the non-aqueous electrolyte, expressed in %.

[0049] b represents the mass percentage of propylene carbonate in the non-aqueous electrolyte, expressed as %.

[0050] c represents the mass percentage of fluoroethylene carbonate in the non-aqueous electrolyte, expressed as %.

[0051] In existing non-aqueous electrolytes for sodium-ion batteries, sodium monofluorophosphate (SFC) is extremely difficult to dissolve. Insufficient SFC content in the non-aqueous electrolyte affects the formation of the electrode interface film, reducing the battery's cycle performance. While the organic solvent propylene carbonate can improve the solubility of SFC, excessive propylene carbonate in the non-aqueous electrolyte leads to a decrease in the battery's initial efficiency and rate performance. Through extensive research, the inventors discovered that adding an appropriate amount of fluoroethylene carbonate to the non-aqueous electrolyte reduces the propylene carbonate content. Utilizing the similarity-miscibility property of fluoroethylene carbonate and SFC, this approach ensures the solubility of SFC while avoiding the decrease in initial efficiency and rate performance caused by excessive use of propylene carbonate as an organic solvent. The inventors discovered that the mass percentages of sodium monofluorophosphate, propylene carbonate, and fluoroethylene carbonate in a non-aqueous electrolyte affect the film-forming quality of the electrolyte interface membrane, the transport rate of sodium ions on the interface membrane, the conductivity of the non-aqueous electrolyte, and the viscosity of the non-aqueous electrolyte under low-temperature conditions, thus leading to changes in the battery's cycle performance, initial efficiency, and rate performance. Specifically, when the ratio d = b / c of ​​the mass percentages of sodium monofluorophosphate (a%), propylene carbonate (c%), and fluoroethylene carbonate (b%) in the non-aqueous electrolyte satisfies the conditions 0.6 ≤ a × d ≤ 15 and 3 ≤ d ≤ 10, the synergistic effect between sodium monofluorophosphate, propylene carbonate, and fluoroethylene carbonate can be fully utilized. The added propylene carbonate increases the solubility of sodium monofluorophosphate in the electrolyte, resulting in a significant improvement in its solubility and enabling it to effectively play its role as an additive and participate in film formation. The addition of fluoroethylene carbonate reduces the amount of propylene carbonate used. While ensuring the solubility of sodium monofluorophosphate, it also avoids the decline in initial efficiency and rate performance caused by excessive use of propylene carbonate solvent. At the same time, it can effectively improve the conductivity of the electrolyte, ensure that the viscosity meets the requirements at low temperatures and that the low-temperature performance is not degraded. The three factors work together to improve the film quality of the positive and negative electrodes, increase the content of fluorinated compounds in the interfacial film, increase the transport rate of sodium ions on the interfacial film, reduce the impedance and growth rate of the interfacial film, and effectively improve the cycle stability of the battery while improving the rate performance.

[0052] In the non-aqueous electrolyte, the ratio d = b / c of ​​the mass percentage of propylene carbonate (b%) to the mass percentage of fluoroethylene carbonate (c%) satisfies the condition 3 ≤ d ≤ 10. If d is less than 3 or greater than 10, sodium monofluorophosphate cannot dissolve effectively in the non-aqueous electrolyte, affecting the formation of the positive and negative electrode interface film, intensifying the decomposition of the non-aqueous electrolyte, and degrading battery performance. In the non-aqueous electrolyte, the mass percentages a% and d of sodium monofluorophosphate satisfy the condition 0.6 ≤ a × d ≤ 15. If the value of a × d is less than 0.6, sodium monofluorophosphate cannot effectively participate in the formation of the interface film, and the poor quality of the interface film leads to poor stability of the positive and negative electrode interface, resulting in intensified side reactions in the non-aqueous electrolyte, reduced battery initial efficiency, and decreased cycle performance. If the value of a × d is greater than 15, the thickness of the formed interface film increases, the battery impedance increases significantly, the sodium ion transport rate decreases, and the battery's cycle performance and rate performance decrease.

[0053] In some preferred embodiments, the ratio d of the mass percentage of propylene carbonate to the mass percentage of fluoroethylene carbonate is in the range of 3 ≤ d ≤ 10.

[0054] In some preferred embodiments, the non-aqueous electrolyte satisfies the following condition: 0.9 ≤ a × d ≤ 12.

[0055] In a preferred embodiment, sodium monofluorophosphate, in synergy with fluoroethylene carbonate and propylene carbonate, effectively participates in the formation of the positive and negative electrode interfacial film, improves the stability of the interfacial film, and the formed interfacial film can effectively inhibit the decomposition of non-aqueous electrolyte, increase the transport rate of sodium ions on the interfacial film, reduce the interfacial film impedance and growth rate, improve the conductivity of non-aqueous electrolyte, and effectively improve the battery's initial efficiency, cycle performance and rate performance.

[0056] The mass percentage (a%) of sodium monofluorophosphate in the non-aqueous electrolyte ranges from 0.1% to 3%. When added as an additive to the non-aqueous electrolyte for sodium-ion batteries, sodium monofluorophosphate forms a film at the positive or negative electrode interface during charge and discharge. This interfacial film inhibits the decomposition of the non-aqueous electrolyte caused by contact between the non-aqueous electrolyte and the active materials of the positive or negative electrodes, significantly improving the stability of the electrodes and the non-aqueous electrolyte, reducing irreversible loss of active sodium, enhancing cycle reversibility, and improving the cycle stability of the battery. If the mass percentage of sodium monofluorophosphate in the non-aqueous electrolyte is less than 0.1%, it cannot effectively participate in film formation, resulting in poor interfacial film quality, poor stability of the positive and negative electrode interfaces, exacerbated side reactions in the non-aqueous electrolyte, capacity decay, and reduced battery cycle performance. If the mass percentage of sodium monofluorophosphate in the non-aqueous electrolyte is greater than 3%, the thickness of the interfacial film increases, battery impedance increases, and battery cycle performance deteriorates. Specifically, the mass percentage a% of sodium monofluorophosphate in the non-aqueous electrolyte can be 0.1%, 0.5%, 0.8%, 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.3%, 2.5%, 2.7%, 2.9%, 3.0%, etc.

[0057] In some preferred embodiments, the mass percentage (a%) of sodium monofluorophosphate in the non-aqueous electrolyte ranges from 0.2% to 1.5%. Within this preferred range, sodium monofluorophosphate can form a stable interfacial film at the positive or negative electrode interface during battery charging and discharging, inhibiting the decomposition of the non-aqueous electrolyte, reducing the occurrence of battery side reactions, and improving battery cycle performance.

[0058] In non-aqueous electrolytes, when the mass percentage (b%) of the organic solvent propylene carbonate is in the range of 5% ≤ b% ≤ 50%, the solubility of sodium monofluorophosphate is significantly improved due to the relatively high dielectric constant of the non-aqueous electrolyte. This allows sodium monofluorophosphate to effectively function as an additive, synergistically participating in the formation of the interfacial film. Simultaneously, propylene carbonate at a mass percentage (b%) effectively improves the conductivity of the non-aqueous electrolyte, ensuring that the viscosity of the non-aqueous electrolyte does not affect the sodium ion transport rate at low temperatures, thus preventing degradation of the battery's low-temperature performance. If the mass percentage of propylene carbonate is less than 5%, the solubility of sodium monofluorophosphate in the non-aqueous electrolyte decreases, hindering its effective participation in film formation, reducing interfacial film stability, and exacerbating the decomposition of the non-aqueous electrolyte, thereby degrading battery performance. If the propylene carbonate content is higher than 40%, excessive propylene carbonate leads to increased viscosity of the non-aqueous electrolyte, reducing the battery's initial efficiency, rate performance, and low-temperature performance. Specifically, in non-aqueous electrolytes, the mass percentage (b%) of the organic solvent propylene carbonate can be 5%, 10%, 14%, 18%, 20%, 25%, 28%, 30%, 34%, 39%, 40%, 45%, 48%, 50%, etc.

[0059] In some preferred embodiments, the mass percentage b% of the propylene carbonate in the non-aqueous electrolyte ranges from 7% to 30%.

[0060] In non-aqueous electrolytes, when the mass percentage (c%) of the fluoroethylene carbonate in the non-aqueous electrolyte ranges from 1% to 5%, the fluoroethylene carbonate synergistically participates in the formation of the interfacial film with sodium monofluorophosphate, improving the film quality of the positive and negative electrodes, increasing the content of fluorinated compounds in the interfacial film, increasing the transport rate of sodium ions on the interfacial film, reducing the impedance and growth rate of the interfacial film, and improving the battery cycle performance and rate performance. If the mass percentage of fluoroethylene carbonate is less than 1%, the fluoroethylene carbonate cannot effectively synergistically participate in film formation and sodium monofluorophosphate cannot be effectively dissolved, degrading battery performance; if the mass percentage of fluoroethylene carbonate is greater than 5%, the fluoroethylene carbonate excessively participates in film formation, increasing the interfacial film thickness, affecting the sodium ion transport rate, and reducing battery cycle performance. Specifically, the mass percentage (c%) of the fluoroethylene carbonate in the non-aqueous electrolyte can be 1%, 1.2%, 1.5%, 1.8%, 2.0%, 2.4%, 2.8%, 3.0%, 3.5%, 3.7%, 4.2%, 4.5%, 4.8%, 5.0%, etc.

[0061] In some preferred embodiments, the mass percentage (c%) of the fluoroethylene carbonate in the non-aqueous electrolyte ranges from 1.5% to 3%. In preferred embodiments, the fluoroethylene carbonate and sodium monofluorophosphate synergistically participate in the formation of the interfacial film, increasing the content of fluorinated compounds in the interfacial film, increasing the transport rate of sodium ions on the interfacial film, reducing the impedance and growth rate of the interfacial film, and resulting in better cycle performance and rate performance of the battery.

[0062] In some embodiments, the sodium salt includes at least one of sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium trifluoromethanesulfonylimide, sodium trifluoromethanesulfonate, sodium tetrafluoroborate, sodium perchlorate, sodium bis(fluoromethanesulfonate)borate, sodium difluorooxalateborate, sodium hexafluoroarsenate, sodium trifluoroacetate, sodium tetraphenylborate, and sodium bis(trifluoromethanesulfonyl)imide.

[0063] In some preferred embodiments, the sodium salt content is 12% to 14% by mass, based on 100% of the non-aqueous electrolyte.

[0064] Adding sodium salt to the non-aqueous electrolyte ensures high solubility and easy dissociation in non-aqueous organic solvents, thereby ensuring high ionic conductivity and promoting the redox reaction of the battery.

[0065] In some embodiments, the non-aqueous organic solvent includes at least one of cyclic or chain carbonates having 3 to 5 carbon atoms, carboxylic acid esters having 2 to 6 carbon atoms, and cyclic or chain ethers having 4 to 10 carbon atoms.

[0066] In some preferred embodiments, the carbonate solvent includes cyclic carbonates or chain carbonates having 3 to 5 carbon atoms. Cyclic carbonates include, but are not limited to, at least one of ethylene carbonate (EC), vinylene carbonate (VC), ethylene ethylene carbonate (VEC), propylene carbonate (PC), γ-butyrolactone (GBL), and butylene carbonate (BC). Chain carbonates may specifically include, but are not limited to, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and dipropyl carbonate (DPC).

[0067] In some preferred embodiments, the carboxylic acid ester solvent includes carboxylic acid esters with 2 to 6 carbon atoms, including but not limited to at least one of methyl acetate (MA), ethyl acetate (EA), propyl acetate (EP), butyl acetate, and propyl propionate (PP). As a preferred embodiment, the non-aqueous electrolyte of the secondary battery also includes vinylene carbonate (VC), ethylene ethylene carbonate (VEC), and fluoroethylene carbonate (FEC).

[0068] In some preferred embodiments, the ether solvent comprises cyclic or chain ethers having 4 to 10 carbon atoms. The cyclic ethers include, but are not limited to, at least one of 1,3-dioxolane (DOL), 1,4-dioxolane (DX), tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-CH3-THF), and 2-trifluoromethyltetrahydrofuran (2-CF3-THF); the chain ethers include, but are not limited to, at least one of dimethoxymethane (DMM), 1,2-dimethoxyethane (DME), and diethylene glycol dimethyl ether (TEGDME).

[0069] In some embodiments, the additive further includes auxiliary additives, which include at least one of vinylene carbonate, ethylene carbonate, ethylene sulfate, 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, 1,4-butanesulfonate lactone, and difluoroethylene carbonate.

[0070] The mass percentage of the non-aqueous electrolyte is 100%, and the mass percentage of the auxiliary additives is 0.01% to 5%.

[0071] Secondly, this application provides a sodium-ion battery, including a positive electrode, a negative electrode, and the aforementioned non-aqueous electrolyte for sodium-ion batteries.

[0072] The sodium-ion battery provided in this application can fully utilize the synergistic effect between sodium monofluorophosphate, propylene carbonate, and fluoroethylene carbonate during the battery charging and discharging process to form a stable interfacial film at the positive and negative electrode interfaces. This increases the transport rate of sodium ions on the interfacial film, reduces the interfacial film impedance and growth rate, and improves the battery's cycle performance and rate performance. It also suppresses the decomposition of the non-aqueous electrolyte caused by contact between the non-aqueous electrolyte and the positive or negative electrode, improves the stability of the electrodes and the non-aqueous electrolyte, and enhances the stability of the battery's first-efficiency performance and cycle performance. Furthermore, it improves the conductivity of the non-aqueous electrolyte, ensuring that the viscosity of the non-aqueous electrolyte meets the requirements at low temperatures and guaranteeing the battery's low-temperature performance.

[0073] In some embodiments, the positive electrode includes a positive electrode active material selected from at least one of layered transition metal oxides, Prussian compounds, phosphate compounds, and sulfate compounds.

[0074] In some embodiments, the negative electrode includes a negative electrode active material, which includes at least one of hard carbon, soft carbon, carbon nanotubes, graphite, graphene, expanded graphite, or mesophase carbon microspheres.

[0075] In some embodiments, the layered transition metal oxide comprises a compound represented by Formula I:

[0076] Na x M y O z Formula I

[0077] Wherein, 0 < x ≤ 1, 0 < y ≤ 1, 1 < z ≤ 2, and M is selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V;

[0078] The Prussian-type compounds include those represented by Formula II:

[0079] Na x′ L y′ [L′(CN)6] y′ ·z′H2O Formula II

[0080] Wherein, 0 < x′ ≤ 2, 0 < y′ ≤ 1, 0 < z′ ≤ 20, and L and L′ are each selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V;

[0081] The phosphate compounds include at least one of the compounds represented by Formula III or Formula IV:

[0082] Na3(M′O 1-q PO4)2F 1+2q Formula III

[0083] Where 0≤q≤1, M′ is selected from at least one of Al, V, Ge, Fe, and Ga:

[0084] Na2EPO4F Formula IV

[0085] Wherein, E is selected from at least one of Fe and Mn;

[0086] The sulfate compounds include at least one of the compounds represented by Formula V;

[0087] Na2Y(SO4)2·2H2O Formula V

[0088] Wherein, Y is selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V.

[0089] In some preferred embodiments, the layered transition metal oxide includes NaNi. m Fe n Mn p O2 compounds, NaNi m Co n Mn p At least one of the O2 compounds, wherein m+n+p=1, 0≤m≤1, 0≤n≤1, 0≤p≤1.

[0090] In some preferred embodiments, the Prussian-like compound includes Na. x′ Mn[Fe(CN)6] y′ ·z′H2O compounds, Na x′ Fe[Fe(CN)6] y′ At least one of the compounds z′H2O, wherein 0<x′≤2, 0<y′≤1, and 0<z′≤20;

[0091] In some preferred embodiments, the phosphate compound includes at least one of Na3(VPO4)2F3, Na3(VOPO4)2F, Na2FePO4F, and Na2MnPO4F.

[0092] The present invention will be further illustrated by the following examples.

[0093] Example 1

[0094] This embodiment is used to illustrate the non-aqueous electrolyte for sodium-ion batteries and the sodium-ion battery disclosed in this application.

[0095] Non-aqueous electrolyte: Propylene carbonate, methyl ethyl carbonate, and diethyl carbonate were mixed in a mass ratio of 30:30:40, and then sodium monofluorophosphate (Na2PO3F) and fluoroethylene carbonate (FEC) were added. The mass percentage of sodium monofluorophosphate and fluoroethylene carbonate in the non-aqueous electrolyte is shown in Table 1, based on the total weight of the non-aqueous electrolyte as 100%.

[0096] The preparation of a sodium-ion battery includes the following steps:

[0097] (1) Preparation of positive electrode: Take positive electrode active material NaNi in a mass ratio of 93:4:3 1 / 3 Fe 1 / 3 Mn 1 / 3 O2, conductive carbon black Super-P, and binder polyvinylidene fluoride (PVDF) are mixed and then dispersed in an appropriate amount of N-methyl-2-pyrrolidone (NMP) to obtain the positive electrode slurry. The obtained slurry is uniformly coated on both sides of aluminum foil, and after drying, rolling and vacuum drying, aluminum leads are welded on using an ultrasonic welding machine to obtain the positive electrode sheet.

[0098] (2) Preparation of the negative electrode: According to the mass ratio of 94:1:2.5:2.5, take the negative electrode active material with a specific surface area of ​​5m². 2 / g of hard carbon, conductive carbon black Super-P, binder styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) are mixed and then dispersed in an appropriate amount of deionized water to obtain a negative electrode slurry. The slurry is coated on both sides of a copper foil, dried, rolled and vacuum dried, and nickel leads are welded on using an ultrasonic welding machine to obtain a negative electrode sheet.

[0099] (3) Preparation of sodium-ion batteries: A separator is placed between the positive and negative electrode sheets prepared above. Then, the sandwich structure consisting of the positive electrode sheet, negative electrode sheet, and separator is wound up. The wound body is then flattened and placed in an aluminum foil packaging bag. It is vacuum baked at 75°C for 48 hours to obtain the cell to be injected with electrolyte. In a glove box with the dew point controlled below -40°C, the non-aqueous electrolyte prepared above is injected into the cell. After vacuum sealing and standing for 24 hours, it is then formed to obtain the finished sodium-ion battery.

[0100] Examples 2-16 and Comparative Examples 1-12

[0101] The differences between Examples 2-16 and Comparative Examples 1-12 and Example 1 are that the mass percentages of sodium monofluorophosphate, propylene carbonate, and fluoroethylene carbonate in the non-aqueous electrolyte are different, as are the contents of auxiliary additives, as detailed in Table 1. The rest of the preparation method is the same as in Example 1.

[0102] Table 1. Parameters of Non-Aqueous Electrolytes for Examples and Comparative Examples

[0103]

[0104]

[0105] Performance testing

[0106] The sodium-ion batteries prepared in the above embodiments and comparative examples were subjected to the following performance tests.

[0107] (1) Conductivity test of non-aqueous electrolyte at 25℃: The non-aqueous electrolytes of various sodium-ion batteries prepared were tested at 25℃ using a conductivity meter.

[0108] (2) Viscosity test of non-aqueous electrolyte at -20℃: The non-aqueous electrolytes of each sodium-ion battery prepared were tested at -20℃ using a viscosity tester.

[0109] (3) High-temperature cycling performance test at 45℃:

[0110] The sodium-ion battery was left to stand at 45°C for 2 hours, then charged at a constant current rate of 0.5C to 3.95V, followed by constant voltage charging to a current of 0.03C, and finally discharged at a constant current rate of 1C to 1.5V. The discharge capacity D1 and the battery internal resistance R1 were recorded. This charge-discharge cycle was repeated 200 times, and the discharge capacity D2 and the battery internal resistance R2 were recorded for the 200th discharge.

[0111] The calculation method is as follows:

[0112] High-temperature cycling capacity retention (%) = D2 / D1 × 100%.

[0113] Internal resistance growth rate (%) = (R2-R1) / R1×100%.

[0114] (4) Cyclic performance test at 25℃:

[0115] The sodium-ion battery was left to stand at 25°C for 2 hours, then charged at a constant current rate of 0.5C to 3.95V, followed by constant voltage charging to a current of 0.03C, and finally discharged at a constant current rate of 1C to 1.5V. The discharge capacity D3 was recorded. This charge-discharge cycle was repeated 200 times, and the discharge capacity D4 of the 200th discharge was recorded.

[0116] The calculation method is as follows: room temperature cycling capacity retention rate (%) = D4 / D3 × 100%.

[0117] (5) Low-temperature discharge test at -20℃:

[0118] The sodium-ion battery was left to stand at 25°C for 2 hours, then charged at a constant current rate of 0.5C to 3.95V, followed by constant voltage charging to a current of 0.03C, and then discharged at a constant current rate of 0.3C to 1.5V. The discharge capacity D5 was recorded. The battery was then charged at a constant current rate of 0.5C to 3.95V, followed by constant voltage charging to a current of 0.03C. Finally, the battery was left to stand at -20°C for 5 hours, and then discharged at a constant current rate of 0.5C to 1.5V. The discharge capacity D6 was recorded.

[0119] The calculation method is as follows:

[0120] Low-temperature discharge capacity retention rate (%) = D6 / D5 × 100%.

[0121] (6) First-efficiency and 4C high-rate discharge tests:

[0122] The sodium-ion battery was left to stand at 25°C for 2 hours, then charged at a constant current rate of 0.5C to 3.95V, followed by constant voltage charging to a current of 0.03C. The charging capacity D7 was recorded. Then, it was discharged at a constant current rate of 0.2C to 1.5V, and the discharge capacity D8 was recorded. The same process was repeated: charging at a constant current rate of 0.5C to 3.95V, followed by constant voltage charging to a current of 0.03C, and then discharging at a constant current rate of 4C to 1.5V, and the discharge capacity D9 was recorded.

[0123] The calculation method is as follows:

[0124] First-efficacy (%) = D8 / D7 × 100%

[0125] 4C rate discharge capacity retention (%) = D9 / D8 × 100%.

[0126] The battery performance test results for the examples and comparative examples are shown in Tables 2 and 3.

[0127] Table 2 Electrical performance test data for Examples 1-13 and Comparative Examples 1-12

[0128]

[0129]

[0130] As shown in Tables 1 and 2, compared with Comparative Examples 1-3, the non-aqueous electrolytes in Comparative Examples 1-3 either lacked sodium monofluorophosphate or had a sodium monofluorophosphate content (a) that was not within the range of 0.1% to 3%. The non-aqueous electrolytes exhibited low conductivity, high viscosity at low temperatures, and high internal resistance growth rates, resulting in low cycle performance and rate performance of the batteries. This indicates that the content of sodium monofluorophosphate in the non-aqueous electrolyte affects the formation of the positive and negative electrode interfacial film. When the content of sodium monofluorophosphate is too low, it affects the formation of the interfacial film; when the content of sodium monofluorophosphate is too high, it participates extensively in film formation, increasing battery impedance and affecting battery cycle performance, low-temperature performance, and rate performance.

[0131] A comparison of Examples 1-13 and Comparative Examples 1-7 shows that as long as the mass percentages of sodium monofluorophosphate (a), propylene carbonate (b), and fluoroethylene carbonate (c) are not within the scope of this application, even if a, b, and c satisfy the relationship d = b / c, 3 ≤ d ≤ 10, and the condition 0.6 ≤ a × d ≤ 15, the battery exhibits low conductivity, high internal resistance growth rate, and high electrolyte viscosity at low temperatures, resulting in poor cycle performance, rate performance, and low-temperature performance. This indicates that the values ​​of a, b, and c are strongly correlated with improving battery cycle performance, rate performance, and low-temperature performance. The three components—sodium monofluorophosphate, propylene carbonate, and fluoroethylene carbonate—work synergistically to participate in the formation of the positive and negative electrode interface film, forming a stable interface film at the positive and negative electrode interface. This increases the transport rate of sodium ions on the interface film, reduces the interface film impedance and growth rate, and improves the battery's cycle performance and rate performance. It also inhibits the decomposition of the non-aqueous electrolyte caused by contact between the non-aqueous electrolyte and the positive or negative electrode, improving the stability of the electrode and the non-aqueous electrolyte, and enhancing the stability of the battery's first-efficiency performance and cycle performance. Furthermore, it improves the conductivity of the non-aqueous electrolyte, ensuring that the viscosity of the non-aqueous electrolyte meets the requirements at low temperatures and guaranteeing the battery's low-temperature performance.

[0132] A comparison of Examples 1-13 and Comparative Examples 8-12 shows that the mass percentage content 'a' of sodium monofluorophosphate meets the range of 0.1% to 3%, the mass percentage content 'b' of propylene carbonate meets the range of 5% to 50%, and the mass percentage content 'c' of fluoroethylene carbonate meets the range of 1% to 5%. However, 'd' = b / c, 'd' is not in the range of 3 to 10, and / or the battery does not meet the condition 0.6 ≤ a × d ≤ 15. This results in reduced low-temperature performance, cycle performance, and rate performance of the battery, and a low initial efficiency. This indicates that the values ​​of 'a', 'b', and 'c' must not only meet the condition... To fully realize the synergistic effect between sodium monofluorophosphate, propylene carbonate, and fluoroethylene carbonate, the following conditions must be met: 1. The ratio of b to c, d = b / c, must satisfy the condition 3 ≤ d ≤ 10, and 2.6 ≤ a × d ≤ 15. This will improve the film quality of the positive and negative electrodes, increase the content of fluorine-containing compounds in the interfacial film, increase the transport rate of sodium ions on the interfacial film, reduce the impedance and growth rate of the interfacial film, improve the cycle performance and rate performance of the battery, and ensure the normal operation of the battery's first efficiency.

[0133] Table 3 Electrical performance test data for Examples 1 and 14-16

[0134]

[0135] As shown in Tables 1 and 3, compared with Examples 14-16, the addition of auxiliary additives VC (ethylene carbonate), PS (1,3-propanesulfonate lactone), and DFEC (difluoroethylene carbonate) to the electrolyte resulted in similar cycle performance, rate performance, low-temperature performance, electrolyte conductivity, low-temperature viscosity, and battery internal resistance growth rate to Example 1. This indicates that the addition of auxiliary additives to the electrolyte synergistically improves the battery's cycle performance, rate performance, and low-temperature performance, ensuring the normal performance of the battery's initial efficiency.

[0136] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A non-aqueous electrolyte for sodium-ion batteries, characterized in that, The non-aqueous electrolyte comprises a non-aqueous organic solvent, a sodium salt, and additives. The non-aqueous organic solvent includes propylene carbonate, and the additives include sodium monofluorophosphate and fluoroethylene carbonate. The non-aqueous electrolyte meets the following conditions: 0.6 ≤ a × d ≤ 15, and d = b / c; Where 0.1≤a≤3, 5≤b≤50, 1≤c≤5, 3≤d≤10; a represents the mass percentage of sodium monofluorophosphate in the non-aqueous electrolyte, expressed in % . b represents the mass percentage of propylene carbonate in the non-aqueous electrolyte, expressed as % . c represents the mass percentage of fluoroethylene carbonate in the non-aqueous electrolyte, in units of 1.

2. The non-aqueous electrolyte for sodium-ion batteries according to claim 1, characterized in that, The ratio d of the mass percentage of propylene carbonate to the mass percentage of fluoroethylene carbonate is in the range of 3 ≤ d ≤ 10. The non-aqueous electrolyte meets the following conditions: 0.9≤a×d≤12.

3. The non-aqueous electrolyte for sodium-ion batteries according to claim 1, characterized in that, The mass percentage (a%) of the sodium monofluorophosphate in the non-aqueous electrolyte ranges from 0.2% to 1.5%.

4. The non-aqueous electrolyte for sodium-ion batteries according to claim 1, characterized in that, The mass percentage (b%) of the propylene carbonate in the non-aqueous electrolyte ranges from 7% to 30%.

5. The non-aqueous electrolyte for sodium-ion batteries according to claim 1, characterized in that, The mass percentage (c%) of the fluoroethylene carbonate in the non-aqueous electrolyte ranges from 1.5% to 3%.

6. The non-aqueous electrolyte for sodium-ion batteries according to claim 1, characterized in that, The sodium salt includes at least one of sodium hexafluorophosphate, sodium difluorosulfonylimide, sodium trifluoromethanesulfonylimide, sodium trifluoromethanesulfonate, sodium tetrafluoroborate, sodium perchlorate, sodium bis(trifluoromethanesulfonyl)imide, sodium difluorooxalateborate, sodium hexafluoroarsenate, sodium trifluoroacetate, sodium tetraphenylborate, and sodium bis(trifluoromethanesulfonyl)imide.

7. The non-aqueous electrolyte for sodium-ion batteries according to claim 6, characterized in that, Based on the mass of the non-aqueous electrolyte as 100%, the mass percentage of the sodium salt is 12% to 14%.

8. The non-aqueous electrolyte for sodium-ion batteries according to claim 1, characterized in that, The non-aqueous organic solvent also includes at least one of the following: cyclic or chain carbonates with 3 to 5 carbon atoms, carboxylic acid esters with 2 to 6 carbon atoms, and cyclic or chain ethers with 4 to 10 carbon atoms.

9. The non-aqueous electrolyte for sodium-ion batteries according to claim 1, characterized in that, The additives also include auxiliary additives, which include at least one of vinylene carbonate, ethylene carbonate, ethylene sulfate, 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, 1,4-butanesulfonate lactone, and difluoroethylene carbonate.

10. The non-aqueous electrolyte for sodium-ion batteries according to claim 9, characterized in that, The mass percentage of the non-aqueous electrolyte is 100%, and the mass percentage of the auxiliary additives is 0.01% to 5%.

11. A sodium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, and a non-aqueous electrolyte for sodium-ion batteries as described in any one of claims 1-10.

12. The sodium-ion battery according to claim 11, characterized in that, The positive electrode includes a positive electrode active material, which includes at least one of layered transition metal oxides, Prussian compounds, phosphate compounds, and sulfate compounds; the negative electrode includes a negative electrode active material, which includes at least one of hard carbon, soft carbon, carbon nanotubes, graphite, graphene, expanded graphite, or mesophase carbon microspheres.

13. The sodium-ion battery according to claim 12, characterized in that, The layered transition metal oxide includes compounds represented by Formula I: Na x M y O z Formula I Wherein, 0 < x ≤ 1, 0 < y ≤ 1, 1 < z ≤ 2, and M is selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V; The Prussian-type compounds include those represented by Formula II: Na x´ L y´ [L´(CN)6] y´ •z´H2O Formula II Wherein, 0<x´≤2, 0<y´≤1, 0<z´≤20, and L and L´ are each selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V; The phosphate compound includes at least one of the compounds represented by Formula III or Formula IV: Na3(M´O 1-q PO4)2F 1+2q Formula III Where 0≤q≤1, M´ is selected from at least one of Al, V, Ge, Fe, and Ga; Na2EPO4F Formula IV Wherein, E is selected from at least one of Fe and Mn; The sulfate compounds include at least one of the compounds represented by Formula V: Na2Y(SO4)2•2H2O (Formula V) Wherein, Y is selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V.

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