A sodium-ion secondary battery

Abstract

In order to overcome the problems of low cycle performance and poor rate performance of the existing sodium ion battery, the application provides a sodium ion secondary battery, which comprises a positive electrode, a negative electrode and a non-aqueous electrolyte, the negative electrode comprises a conductive agent, a binder and a negative electrode active material, the non-aqueous electrolyte comprises a sodium salt, an additive and a non-aqueous organic solvent, the additive comprises sodium bisfluorosulfonylimide, and the non-aqueous organic solvent comprises propylene carbonate; the sodium ion secondary battery satisfies the following relationship: wherein 1% <= a% <= 10%, 2% <= b% <= 10%, 2 mu m <= c <= 12 mu m, 1% <= x% <= 5%, and 5% <= y% <= 40%; a% is the mass content of the conductive agent, b% is the mass content of the binder, c is the particle size of the negative electrode active material, x% is the mass content of sodium bisfluorosulfonylimide, and y% is the mass content of propylene carbonate; the sodium ion secondary battery provided by the application can significantly improve the cycle performance and high rate performance of the secondary battery.

Description

Technical Field

[0001] This invention belongs to the field of energy storage battery technology, specifically relating to a sodium-ion secondary battery. Background Technology

[0002] Sodium-ion batteries and lithium-ion batteries started development almost simultaneously. Compared to lithium, sodium resources account for approximately 2.64% of the Earth's crustal element reserves, and their acquisition is relatively simple. The working principle of sodium-ion batteries is similar to that of lithium-ion batteries, utilizing the insertion and extraction of sodium ions between the positive and negative electrodes to achieve charging and discharging. Compared to lithium batteries, sodium-ion batteries have abundant resources, lower costs, and less price fluctuations, and their wide temperature range and high safety performance give them potential as a substitute. 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 vast growth potential. Therefore, developing high-performance, low-cost sodium-ion batteries is a decisive factor in their industrialization.

[0003] Currently, sodium-ion batteries contain sodium ions in the positive electrode. Because sodium ions have a larger radius than lithium ions, they suffer from poor rate performance and low cycle performance during charge and discharge. Research on sodium-ion batteries mainly focuses on adding film-forming additives to the electrolyte to improve high-temperature cycle performance, and some studies improve the positive electrode active material to enhance rate performance. However, research on optimizing the content of electrolyte additives, organic solvents, and negative electrode components to improve the cycle and rate performance of sodium-ion batteries is relatively limited. Summary of the Invention

[0004] To address the problems of low cycle performance and poor rate performance of existing sodium-ion batteries, this invention provides a sodium-ion secondary battery.

[0005] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows:

[0006] This invention provides a sodium-ion secondary battery, comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode comprises a negative electrode active material layer, which includes a conductive agent, a binder, and a negative electrode active material. Based on the mass of the negative electrode active material layer as 100%, the mass content of the conductive agent is a%, the mass content of the binder is b%, and the particle size of the negative electrode active material is c.

[0007] The non-aqueous electrolyte includes additives and a non-aqueous organic solvent. The additives include sodium difluorosulfonamide, and the non-aqueous organic solvent includes propylene carbonate. Based on the total mass of the non-aqueous electrolyte being 100%, the mass content of sodium difluorosulfonamide is x%, and the mass content of propylene carbonate is y%.

[0008] The sodium-ion secondary battery satisfies the following relationship:

[0009]

[0010] Among them, 1%≤a%≤10%, 2%≤b%≤10%, 2μm≤c≤12μm, 1%≤x%≤5%, and 5%≤y%≤40%.

[0011] Preferably, the sodium-ion secondary battery satisfies the following relationship:

[0012]

[0013] Preferably, based on the mass of the negative electrode active material layer being 100%, the mass content a% of the conductive agent is in the range of 2% ≤ a% ≤ 6%.

[0014] Preferably, based on the mass of the negative electrode active material layer as 100%, the mass content b% of the binder ranges from 3% to b% to 6%.

[0015] Preferably, the negative electrode active material includes one or more of hard carbon and soft carbon;

[0016] The particle size c of the negative electrode active material is in the range of 3μm≤c≤9μm.

[0017] Preferably, based on the total mass of the non-aqueous electrolyte as 100%, the mass content x% of the sodium difluorosulfonamide ranges from 1.5% to x% to 4%.

[0018] Preferably, based on the total mass of the non-aqueous electrolyte as 100%, the mass content y% of the propylene carbonate ranges from 10% to y% to 30%.

[0019] Preferably, the non-aqueous organic solvent further includes one or more of the following: carbonate solvents having 3 to 5 carbon atoms, carboxylic acid ester solvents having 2 to 6 carbon atoms, and ether solvents having 4 to 10 carbon atoms.

[0020] Preferably, the additive further includes one or more of sulfate ester compounds and fluorocarbonate compounds;

[0021] Based on the total mass of the non-aqueous electrolyte as 100%, the mass content of the sulfate ester compound is 1% to 3%, and the mass content of the fluorocarbonate compound is 1% to 5%.

[0022] Preferably, the non-aqueous electrolyte further includes a sodium salt, which includes one or more of sodium perchlorate, sodium tetrafluoroborate, sodium hexafluorophosphate, sodium trifluoroacetate, sodium tetraphenylborate, sodium trifluoromethanesulfonate, sodium bis(fluorosulfonyl)imide, and sodium bis(trifluoromethanesulfonyl)imide.

[0023] Based on the mass of the non-aqueous electrolyte as 100%, the mass content of the sodium salt is 8% to 15%.

[0024] Beneficial effects

[0025] Compared with the prior art, the sodium-ion secondary battery provided in this application has the following properties: the mass content a% of the conductive agent in the negative electrode active material layer satisfies 1% ≤ a% ≤ 10%; the mass content b% of the binder in the negative electrode active material layer satisfies 2% ≤ b% ≤ 10%; the particle size c of the negative electrode active material satisfies 2μm ≤ c ≤ 12μm; the mass content x% of the additive sodium difluorosulfonamide in the non-aqueous electrolyte satisfies 1% ≤ x% ≤ 5%; and the mass content y% of the non-aqueous organic solvent propylene carbonate in the non-aqueous electrolyte satisfies 5% ≤ y% ≤ 40%. Furthermore, the sodium-ion secondary battery satisfies the following relationship: The prepared sodium-ion secondary battery has a high transport rate at the negative electrode and high conductivity in the electrolyte, enabling high-rate discharge above 5C. At the same time, the good quality of the SEI film at the negative electrode interface, the high stability of the electrolyte, and the high conductivity of the negative electrode can improve the cycle performance and high-rate performance of the battery. Detailed Implementation

[0026] To make the technical problems solved, the technical solutions, and the beneficial effects of this invention clearer, the 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 and not intended to limit the invention.

[0027] This invention provides a sodium-ion secondary battery, comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode includes a negative electrode active material layer, which comprises a conductive agent, a binder, and a negative electrode active material. Based on the mass of the negative electrode active material layer being 100%, the mass content of the conductive agent is a%, the mass content of the binder is b%, and the particle size of the negative electrode active material is c.

[0028] The non-aqueous electrolyte includes additives and a non-aqueous organic solvent. The additives include sodium difluorosulfonamide, and the non-aqueous organic solvent includes propylene carbonate. Based on the total mass of the non-aqueous electrolyte being 100%, the mass content of sodium difluorosulfonamide is x%, and the mass content of propylene carbonate is y%.

[0029] The sodium-ion secondary battery satisfies the following relationship:

[0030]

[0031] Among them, 1%≤a%≤10%, 2%≤b%≤10%, 2μm≤c≤12μm, 1%≤x%≤5%, and 5%≤y%≤40%.

[0032] Because sodium ions have a larger ionic radius than lithium ions, their kinetics for sodium ion desodiumation and intercalation during operation are lower, especially at high rates, resulting in even lower high-rate charge-discharge performance. Through extensive research, the inventors discovered that improving the cycle and rate performance of sodium-ion batteries can be achieved by focusing on the negative electrode and electrolyte. This involves limiting the proportion of conductive agents and binders in the negative electrode active material layer, controlling the particle size range of the negative electrode active material, and using an electrolyte with NaFSI (sodium bis(fluorosulfonyl)imide) as an additive and a high content of PC (propylene carbonate) solvent, resulting in an electrolyte with high conductivity and low viscosity. Through extensive experiments, the inventors discovered that the mass content a% of the conductive agent in the negative electrode active material layer satisfies 1% ≤ a% ≤ 10%, the mass content b% of the binder in the negative electrode active material layer satisfies 2% ≤ b% ≤ 10%, the particle size c of the negative electrode active material satisfies 2μm ≤ c ≤ 12μm, the mass content x% of the additive sodium difluorosulfonamide in the non-aqueous electrolyte satisfies 1% ≤ x% ≤ 5%, and the mass content y% of the non-aqueous organic solvent propylene carbonate in the non-aqueous electrolyte satisfies 5% ≤ y% ≤ 40%. Furthermore, the sodium-ion secondary battery satisfies the following relationship: The prepared sodium-ion secondary battery, due to the high transport rate of the negative electrode and the high conductivity of the electrolyte, can achieve high-rate discharge of more than 5C. At the same time, the SEI film has good film formation quality, excellent electrolyte stability and high negative electrode conductivity, resulting in better cycle stability. It can realize the performance advantages of sodium-ion batteries in terms of high rate and long cycle.

[0033] Through extensive experimentation, the inventors discovered that if the value of the sodium-ion secondary battery relationship xy / 10c(a+b) is greater than 3.3, the ion transport rate is low, the battery impedance increases significantly, and the rate performance of the battery decreases; if the value of the relationship xy / 10c(a+b) is less than 0.01, the quality of the SEI film formed at the negative electrode interface is poor, the side reactions increase during the battery charge and discharge cycle, the battery cycle stability decreases, and the battery gas production increases.

[0034] Specifically, the value of the relationship xy / 10c(a+b) for sodium-ion secondary batteries can be 0.01, 0.02, 0.04, 0.08, 0.1, 0.14, 0.18, 0.2, 0.25, 0.3, 0.34, 0.4, 0.5, 0.6, 0.7, 0.8, 1.0, 1.2, 1.5, 1.8, 2.0, 2.3, 2.6, 3.0, 3.1, 3.3, etc., and the value of the relationship xy / 10c(a+b) should be in the range of 0.01 to 3.3.

[0035] In some preferred embodiments, the sodium-ion secondary battery satisfies the following relationship: The battery satisfies the relationship between 0.02 and 0.8. The electrolyte has higher conductivity, higher ion transport rate, and lower viscosity. The battery impedance is low. The SEI film formed at the negative electrode interface is of good quality, and the negative electrode has a higher transport rate, which makes the battery have better high-rate performance and long-cycle performance.

[0036] Assuming the mass content of the negative electrode active material layer is 100%, if the mass content of the conductive agent is a%, satisfying 1% ≤ a% ≤ 10%, the negative electrode will have a low resistivity. If the mass content of the conductive agent is less than 1%, the conductivity of the negative electrode active material layer will be low, and the rate performance of the battery will be reduced. If the mass content of the conductive agent is greater than 10%, the mass ratio of the negative electrode active material will decrease, and the energy density of the battery will decrease.

[0037] In some preferred embodiments, the mass content a% of the conductive agent ranges from 2% to a% to 6%.

[0038] Within this preferred range, the mass content of the conductive agent, a%, results in a battery with high energy density and high conductivity, which satisfies the kinetic response requirements for the battery's rate performance.

[0039] Specifically, the mass content a% of the conductive agent can be 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.4%, 4.0%, 4.6%, 5.0%, 5.5%, 6.0%, 7.0%, 8.0%, 9.0%, or 10.0%, as long as the mass content of the conductive agent is within the range of 1% to 10%.

[0040] Assuming the negative electrode active material layer has a mass content of 100%, the binder mass content b% must satisfy 2% ≤ b% ≤ 10%. Specifically, the binder mass content b% can be 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.6%, 5.0%, 5.5%, 6.0%, 6.5%, 7.0%, 7.5%, 8.0%, 8.5%, 9.0%, or 10.0%, as long as it is within the range of 2% to 10%. If the binder mass content of the negative electrode active material layer is less than 2%, the bonding effect of the negative electrode active material particles in the negative electrode active material layer is poor, and the negative electrode active material layer is prone to detaching from the current collector, resulting in poor cycle performance and rate performance of the battery. If the binder mass content is greater than 10%, the mass proportion of the negative electrode active material decreases, and the energy density of the battery decreases. The binder content b% in the negative electrode active material layer is within the range of 2% ≤ b% ≤ 10%, which results in good bonding of the negative electrode active material particles and strong adhesion between the negative electrode active material layer and the current collector, thus ensuring improved battery cycle performance and rate performance.

[0041] In some preferred embodiments, the mass content b% of the adhesive ranges from 3% to b% to 6%.

[0042] The inventors discovered that the conductive agent and binder in the negative electrode active material layer are within the scope of the present invention. The negative electrode has both low resistivity and better adhesion between particles of the negative electrode active material. The electrode material has better stability during battery testing, which is conducive to the formation of an SEI film on the surface of the negative electrode active material layer. The battery has better rate discharge and cycle performance.

[0043] In some embodiments, the negative electrode active material includes one or more of hard carbon and soft carbon; the particle size c of the negative electrode active material is in the range of 2μm≤c≤12μm; here, particle size c is the particle size range corresponding to D50. Multiple experiments have shown that negative electrode active materials with a particle size D50 within this range balance a relatively fast ion transport rate with avoiding excessive electrolyte decomposition and participation in film formation, thereby ensuring good rate performance of the battery while improving the battery's first charge-discharge efficiency. Specifically, the particle size c of the negative electrode active material can be 2.0μm, 2.5μm, 3.0μm, 3.5μm, 4.0μm, 5.0μm, 6.0μm, 7.2μm, 8.1μm, 9.0μm, 10.0μm, 11.0μm, or 12.0μm, as long as the particle size c of the negative electrode active material is in the range of 2μm≤c≤12μm.

[0044] In some preferred embodiments, the particle size c of the negative electrode active material is in the range of 3μm≤c≤9μm.

[0045] Based on the total mass of the non-aqueous electrolyte as 100%, the mass content of sodium difluorosulfonamide in the range of 1% ≤ x% ≤ 5%.

[0046] Adding sodium bis(fluorosulfonyl)imide (NaFSI) to the electrolyte can improve its conductivity, participate in the formation of the SEI film at the negative electrode interface, and effectively improve the battery's cycle performance. If the NaFSI content in the electrolyte exceeds 5%, the electrolyte viscosity increases, and high NaFSI content easily corrodes the current collector, leading to the phenomenon of the active material layer of the electrode detaching from the current collector, thus degrading battery performance. If the NaFSI content in the electrolyte is less than 1%, the electrolyte conductivity is too low, and it cannot participate in the formation of the negative electrode SEI film.

[0047] In some preferred embodiments, the mass content of sodium bis(fluorosulfonyl)imide (NaFSI) is in the range of 1.5% ≤ x% ≤ 4%, based on the total mass of the non-aqueous electrolyte as 100%.

[0048] Specifically, the mass content x% of sodium bis(fluorosulfonyl)imide (NaFSI) can be 1.0%, 1.5%, 2.0%, 2.6%, 3.0%, 3.7%, 4.0%, 4.2%, 5.0%, etc., as long as the mass content x% of sodium bis(fluorosulfonyl)imide (NaFSI) is in the range of 1% ≤ x% ≤ 5%.

[0049] Based on the total mass of the non-aqueous electrolyte as 100%, the mass content y% of the propylene carbonate ranges from 5% to y% to 40%.

[0050] The presence of a high content of propylene carbonate (a non-aqueous organic solvent) in the electrolyte can improve its conductivity and enhance the battery's rate performance. If the mass content of propylene carbonate is less than 5%, it cannot effectively improve the electrolyte's conductivity, thus failing to improve the battery's rate performance. If the mass content of propylene carbonate in the electrolyte is higher than 40%, the electrolyte viscosity increases, and this viscosity increases significantly at low temperatures, degrading battery performance. Specifically, the mass content of propylene carbonate in the electrolyte can be 5%, 8%, 10%, 12%, 13%, 15%, 17%, 19%, 22%, 24%, 25%, 28%, 30%, 33%, 35%, 38%, or 40%, as long as the mass content is between 5% and 40%.

[0051] In some embodiments, based on the total mass of the non-aqueous electrolyte as 100%, the mass content y% of the propylene carbonate ranges from 10% to y% to 30%.

[0052] It should be noted that the non-aqueous electrolyte in this application needs to contain both propylene carbonate (PC) and NaFSI (sodium bis(fluorosulfonyl)imide). NaFSI, as an additive and a high-content PC solvent, can improve the conductivity, electrochemical performance and thermal stability of the electrolyte, participate in the formation of the SEI film, reduce side reactions, effectively reduce the impedance during battery cycling, and improve cycle performance.

[0053] In some embodiments, the non-aqueous electrolyte further includes a sodium salt, which includes one or more of sodium perchlorate, sodium tetrafluoroborate, sodium hexafluorophosphate, sodium trifluoroacetate, sodium tetraphenylborate, sodium trifluoromethanesulfonate, sodium bis(fluorosulfonyl)imide, and sodium bis(trifluoromethanesulfonyl)imide.

[0054] Based on the mass of the non-aqueous electrolyte as 100%, the mass percentage of sodium salt in the non-aqueous electrolyte is 8% to 15%.

[0055] In non-aqueous electrolytes, sodium salts dissociate to form sodium metal ions, which intercalate and deintercalate between the positive and negative electrodes to complete the charge-discharge cycle. The mass content of sodium salts directly affects the transfer rate of sodium metal ions, which in turn affects the potential change at the negative electrode. When the sodium salt content is too low, the intercalation and deintercalation efficiency of sodium metal ions between the positive and negative electrodes is reduced, failing to meet the requirements of fast charging. When the sodium salt content is too high, it leads to an increase in the viscosity of the non-aqueous electrolyte, which is also detrimental to improving the intercalation and deintercalation efficiency of sodium metal ions, thus increasing the internal resistance of the battery.

[0056] In some embodiments, the additive further includes one or more of sulfate ester compounds and fluorocarbonate compounds;

[0057] Based on the total mass of the non-aqueous electrolyte as 100%, the mass content of the sulfate ester compound is 1% to 3%, and the mass content of the fluorocarbonate compound is 1% to 5%.

[0058] Specifically, based on the total mass of the non-aqueous electrolyte as 100%, the mass content of sulfate ester compounds can be 1.0%, 1.2%, 1.5%, 1.7%, 1.8%, 2.0%, 2.3%, 2.5%, 2.6%, 2.8%, 3.0%, etc., as long as the mass content of sulfate ester compounds is within the range of 1% to 3%. Based on the total mass of the non-aqueous electrolyte as 100%, the mass content of fluorocarbonate compounds can be 1.0%, 1.2%, 1.5%, 1.7%, 1.8%, 2.0%, 2.3%, 2.5%, 2.6%, 2.8%, 3.0%, 3.2%, 3.4%, 3.5%, 3.6%, 3.8%, 3.9%, 4.0%, 4.3%, 4.4%, 4.5%, 4.7%, 4.8%, 4.9%, 5.0%, etc., as long as the mass content of fluorocarbonate compounds is within the range of 1% to 5%.

[0059] The sulfate compounds include at least one of RPS (1,3-propenesulfonate lactone) and DTD (ethylene sulfate); the fluorocarbonate compounds include one or more of fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC).

[0060] In the non-aqueous electrolyte, compared to adding a single compound or a combination of other existing additives, sodium bis(fluorosulfonyl)imide, when added together with sulfate compounds and fluorocarbonate compounds, exhibits a significant synergistic effect in improving battery performance. This indicates that the co-formation of a film on the electrode surface by sodium bis(fluorosulfonyl)imide, sulfate compounds, and fluorocarbonate compounds can compensate for the film-forming defects of single additions, resulting in a more stable SEI film.

[0061] In some embodiments, the non-aqueous organic solvent further includes one or more of the following: carbonate solvents having 3 to 5 carbon atoms, carboxylic acid ester solvents having 2 to 6 carbon atoms, and ether solvents having 4 to 10 carbon atoms.

[0062] Based on the total mass of the non-aqueous electrolyte being 100%, the mass content of the non-aqueous organic solvent is 70% to 92%.

[0063] In some embodiments, ether solvents having 4 to 10 carbon atoms include cyclic ethers or chain ethers. The cyclic ethers may specifically be, but are not limited to, one or more of 1,3-dioxolane (DOL), 1,4-dioxolane (DX), crown ethers, tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-CH3-THF), and 2-trifluoromethyltetrahydrofuran (2-CF3-THF). The chain ethers may specifically be, but are not limited to, dimethoxymethane, diethoxymethane, ethoxymethoxymethane, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, and diethylene glycol dimethyl ether. Because chain ethers have high solvation ability with sodium ions and can improve ion dissociation, dimethoxymethane, diethoxymethane, and ethoxymethoxymethane, which have low viscosity and can impart high ionic conductivity, are particularly preferred. Ether compounds may be used alone or in any combination and proportion of two or more.

[0064] In some embodiments, carbonate solvents having 3 to 5 carbon atoms include, but are not limited to, one or more of ethylene carbonate (EC), γ-butyrolactone (GBL), butylene carbonate (BC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and dipropyl carbonate (DPC).

[0065] The organic solvent content is 70% to 92%. By setting this range, the conductivity can be avoided due to the decrease in the dielectric constant of the non-aqueous electrolyte, and the high-current discharge characteristics, stability relative to the negative electrode, and cycle characteristics of the non-aqueous electrolyte battery can be made to reach a good range.

[0066] In some embodiments, fluorine-containing chain carbonates (hereinafter referred to as "fluorinated chain carbonates") are also preferably used. There is no particular limitation on the number of fluorine atoms in a fluorinated chain carbonate as long as it is 1 or more, but it is generally 6 or less, preferably 4 or less. When a fluorinated chain carbonate has multiple fluorine atoms, these fluorine atoms can be bonded to the same carbon atom or to different carbon atoms. Examples of fluorinated chain carbonates include dimethyl fluorinated carbonate derivatives, methyl ethyl fluorinated carbonate derivatives, and diethyl fluorinated carbonate derivatives.

[0067] In some embodiments, the carboxylic acid ester solvents having 2 to 6 carbon atoms include cyclic carboxylic acid esters and / or linear carbonates. Examples of the cyclic carboxylic acid esters may include one or more of γ-valerolactone, δ-valerolactone. Examples of the linear carbonates may include one or more of methyl acetate (MA), ethyl acetate (EA), propyl acetate (EP), butyl acetate, propyl propionate (PP), butyl propionate.

[0068] In some embodiments, the positive electrode includes a positive electrode active material layer, and the positive electrode active material layer includes a positive electrode active material. The type and content of the positive electrode active material are not particularly limited and can be selected according to actual needs, as long as it is a positive electrode active material or a conversion-type positive electrode active material that can reversibly intercalate / deintercalate sodium ions.

[0069] In some embodiments, the positive electrode active material is selected from sodium-containing layered oxides, sodium-containing polyanion compounds, and sodium-containing Prussian blue compounds.

[0070] In a preferred embodiment, the positive electrode active material includes but is not limited to at least one of transition metal oxides, Prussian materials, phosphates, sulfates, and titanate materials. Among them, the chemical formula of the transition metal oxide can be Na z M x O y , 0 < x ≤ 1, 0 < y ≤ 1, 1 < z ≤ 2, M can be selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, V. More preferably, the transition metal oxide is NaNi m Fe n＇ Mn p O2 (m + n'+ p = 1, 0 ≤ m ≤ 1, 0 ≤ n' ≤ 1, 0 ≤ p ≤ 1) or NaNi m Co n Mn p O2 (m + n + p = 1, 0 ≤ m ≤ 1, 0 ≤ n ≤ 1, 0 ≤ p ≤ 1); the chemical formula of the Prussian material is Na x′ M′′[M′(CN)6] y＇ ·z′H2O, where M″ is a transition metal, M′ is a transition metal, 0 < x′ ≤ 2, 0 ≤ y′ < 1, 0 < z′ ≤ 20. More preferably, the Prussian material is Na x′ Mn[Fe(CN)6] y′ ·n′H2O (0 < x′ ≤ 2, 0 < y′ ≤ 1, 0 < n′ ≤ 10) or Na x′ Fe[Fe(CN)6] y′ ·n′H2O (0 < x′ ≤ 2, 0 < y′ ≤ 1, 0 < n′ ≤ 20); the chemical formula of the phosphate is Na3(MO1-e PO4)2F 1+2e 0≤e≤1, M is selected from at least one of Al, V, Ge, Fe, Ga, more preferably, the phosphate is Na3(VPO4)2F3 or Na3(VOPO4)2F; the chemical formula of the phosphate is Na2MPO4F, M is selected from at least one of Fe and Mn, more preferably, the phosphate is Na2FePO4F or Na2MnPO4F; the titanate material can be selected from Na2Ti3O7, Na2Ti6O 13 Na4Ti5O 12 Li4Ti5O 12 At least one of NaTi2(PO4)3; the sulfate has the chemical formula Na2M(SO4)2·2H2O, where M can be selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V.

[0071] In some embodiments, the positive electrode further includes a positive electrode current collector, and the positive electrode active material layer is disposed on the surface of the positive electrode current collector.

[0072] The positive electrode current collector is selected from a metallic material that can conduct electrons. Preferably, the positive electrode current collector includes at least one of Al, Ni, tin, copper, and stainless steel. In a more preferred embodiment, the positive electrode current collector is selected from aluminum foil.

[0073] In some embodiments, the positive electrode active material layer further includes a positive electrode binder and a positive electrode conductive agent, and the positive electrode active material, the positive electrode binder and the positive electrode conductive agent are blended to obtain the positive electrode active material layer.

[0074] The positive electrode binder includes at least one of the following: polyvinylidene fluoride, copolymers of polyvinylidene fluoride, polytetrafluoroethylene, copolymers of polyvinylidene fluoride-hexafluoropropylene, copolymers of tetrafluoroethylene-hexafluoropropylene, copolymers of tetrafluoroethylene-perfluoroalkyl vinyl ethers, copolymers of ethylene-tetrafluoroethylene, copolymers of polyvinylidene fluoride-tetrafluoroethylene, copolymers of polyvinylidene fluoride-trifluoroethylene, copolymers of polyvinylidene fluoride-trichloroethylene, copolymers of polyvinylidene fluoride-fluorinated vinylidene ether, copolymers of polyvinylidene fluoride-hexafluoropropylene-tetrafluoroethylene, thermoplastic polyimide, thermoplastic resins such as polyethylene and polypropylene, acrylic resins, and styrene-butadiene rubber.

[0075] The positive electrode conductive agent includes at least one of conductive carbon black, conductive carbon spheres, conductive graphite, conductive carbon fiber, carbon nanotubes, graphene, and reduced graphene oxide.

[0076] Its negative electrode active material includes at least one of the following: metallic sodium, graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microspheres, silicon-based materials, tin-based materials, lithium titanate, and other metals that can form alloys with sodium. The alloy material may also be selected from at least one of Si, Ge, Sn, Pb, and Sb combined with C; the graphite may be selected from at least one of artificial graphite, natural graphite, and modified graphite; the silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, and silicon alloys; and the tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys.

[0077] In some embodiments, the negative electrode further includes a negative electrode current collector, and the negative electrode active material layer is disposed on the surface of the negative electrode current collector. The material of the negative electrode current collector is the same as that of the positive electrode current collector, and will not be described again here.

[0078] In some embodiments, the negative electrode active material, the negative electrode binder, and the negative electrode conductive agent are blended to obtain the negative electrode active material layer.

[0079] The negative electrode conductive agent includes at least one of conductive carbon black, conductive carbon spheres, conductive graphite, conductive carbon fiber, carbon nanotubes, graphene, or reduced graphene oxide.

[0080] The negative electrode binder includes at least one of the following: sodium carboxymethyl cellulose, polyvinylidene fluoride, copolymers of polyvinylidene fluoride, polytetrafluoroethylene, copolymers of polyvinylidene fluoride and hexafluoropropylene, copolymers of tetrafluoroethylene and hexafluoropropylene, copolymers of tetrafluoroethylene and perfluoroalkyl vinyl ethers, copolymers of ethylene and tetrafluoroethylene, copolymers of polyvinylidene fluoride and tetrafluoroethylene, copolymers of polyvinylidene fluoride and trifluoroethylene, copolymers of polyvinylidene fluoride and trichloroethylene, copolymers of polyvinylidene fluoride and fluorinated ethylene, copolymers of polyvinylidene fluoride, hexafluoropropylene and tetrafluoroethylene, thermoplastic polyimide, thermoplastic resins such as polyethylene and polypropylene, acrylic resins, and styrene-butadiene rubber.

[0081] In some embodiments, the secondary battery further includes a separator located between the positive electrode and the negative electrode.

[0082] The diaphragm can be a conventional diaphragm, such as a ceramic diaphragm, a polymer diaphragm, a non-woven fabric, or an inorganic-organic composite diaphragm, including but not limited to single-layer PP (polypropylene), single-layer PE (polyethylene), double-layer PP / PE, double-layer PP / PP, and triple-layer PP / PE / PP diaphragms.

[0083] The present invention will be further illustrated by the following examples.

[0084] Example 1

[0085] This embodiment illustrates the sodium-ion secondary battery preparation method disclosed in this invention, including the following steps:

[0086] 1) Preparation of non-aqueous electrolyte:

[0087] Ethylene carbonate (EC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC) were mixed in a mass ratio of EC:DEC:EMC = 1:1:1. Then, sodium hexafluorophosphate (NaPF6) at a mass content of 8% was added, along with the additive sodium bis(fluorosulfonyl)imide (NaFSI) and the organic solvent propylene carbonate (PC). Table 1 shows the content of sodium bis(fluorosulfonyl)imide (NaFSI) and the content of propylene carbonate (PC) as y% of the total weight of the non-aqueous electrolyte (100%). The electrolyte also contained sulfated esters and fluorocarbonate compounds. The sulfated ester was 1,3-propenesulfonate lactone, and the fluorocarbonate was fluoroethylene carbonate, with both compounds having a mass content of 2.0% and 2.0% respectively.

[0088] 2) Preparation of the positive electrode:

[0089] The positive electrode active material (Na) was mixed in a mass ratio of 94:3:3. 1.2 Ni2[Fe(CN)6] 0.5 Conductive carbon black (H2O), Super-P, and polyvinylidene fluoride (PVDF) binder are dispersed in N-methylpyrrolidone (NMP) to obtain a positive electrode slurry. The slurry is uniformly coated on both sides of an aluminum foil, dried, calendered, and vacuum dried. Aluminum or nickel leads are then welded on using an ultrasonic welder to obtain a positive electrode sheet with a thickness between 80-200 μm.

[0090] 3) Preparation of the negative electrode:

[0091] A mixture of conductive agent Super-P (a%), binder carboxymethyl cellulose (CMC) (b%), and negative electrode active material (100-a%-b%) was prepared and dispersed in deionized water. The mixture was stirred until homogeneous to obtain a negative electrode slurry. The particle size of the negative electrode active material was c μm. The slurry was coated onto both sides of an aluminum foil, dried, calendered, and vacuum dried. Aluminum or nickel leads were then welded onto the foil using an ultrasonic welder to obtain a negative electrode sheet with a thickness between 80-300 μm. The specific values ​​of a%, b%, and c are shown in Table 1.

[0092] 4) Cell fabrication

[0093] A separator is placed between the positive and negative electrode sheets prepared above. Then, the sandwich structure composed of the positive electrode sheet, negative electrode sheet and separator is wound up. The wound body is flattened and placed in an aluminum foil packaging bag. It is then vacuum baked at 75°C for 48 hours to obtain the battery cell to be injected with electrolyte.

[0094] 5) Electrolyte injection and formation of battery cells

[0095] In a glove box with the dew point controlled below -40°C, the electrolyte prepared above was injected into the battery cell, vacuum sealed, and left to stand for 24 hours. Then, constant current charging formation was performed according to the following steps: the sodium-ion battery was charged at a constant current of 0.05C for 180 minutes, then charged at a constant current of 0.3C until 3.95V was reached, at which point charging was stopped. The sodium-ion battery was then aged at room temperature for 30 minutes and vacuum sealed again. The sodium-ion battery was then charged and formed at a rate of 0.3C until the formation potential reached 4.2V, at which point charging was stopped. The sodium-ion battery was then aged at room temperature for 24 hours, and the sodium-ion battery was charged and formed at a rate of 0.3C until 100% SOC was reached, thus obtaining the sodium-ion battery.

[0096] Examples and Comparative Examples

[0097] Examples 2-28 and Comparative Examples 1-12 are used to illustrate the sodium-ion battery and its preparation method disclosed in this invention, including most of the operation steps in Example 1, the difference being:

[0098] The non-aqueous electrolyte contains x% of the additive sodium difluorosulfonamide (NaFSI) and y% of the organic solvent propylene carbonate (PC), as shown in Table 1.

[0099] The mass content of the conductive agent (a%), the mass content of the binder (b%), and the particle size of the negative electrode active material (c) are detailed in Table 1. Table 1: Parameters of the Examples and Comparative Examples

[0100]

[0101]

[0102] The sodium-ion batteries prepared in Examples 1-28 and Comparative Examples 1-12 were subjected to the following performance tests:

[0103] 1. Corrosion analysis of aluminum current collectors:

[0104] The batteries of Example 1 and Comparative Example 3 were charged at a constant current of 1C and a constant voltage of 4.2V, respectively, and then subjected to a 0.03C cut-off charge and a 1C / 1.5V cut-off discharge. The effectiveness of the sodium-ion battery in suppressing NaFSI corrosion of the aluminum current collector was then evaluated at 50 and 200 cycles, respectively, as detailed in Table 2.

[0105] The corrosion area ratio mentioned in the table refers to the ratio of the corroded area to the total current collector area, determined by observing the battery after 200 cycles using a metallographic microscope and a scanning electron microscope.

[0106] 2. The following battery performance tests were conducted on Examples 1-28 and Comparative Examples 1-12:

[0107] (1) 5C rate discharge capacity ratio: The ratio of the capacity released by the battery from 3.95V to 1.5V at 5C rate to the capacity released by the battery at 0.2C rate during the activation phase.

[0108] (2) 25-cycle test

[0109] Charge the battery to 3.9V at a constant current and constant voltage of 0.7C, cut off the current at 0.05C, and then discharge it to 1.5V at a constant current of 1C. Repeat this charge and discharge cycle for 400 cycles. Calculate the cycle capacity retention rate: Cycle capacity retention rate (%) = Discharge capacity after 400 test cycles / Average discharge capacity of the first 3 cycles × 100%.

[0110] (3) High-temperature cycling performance

[0111] The formed sodium-ion battery was charged to 3.9V at 45°C using a constant current and constant voltage of 0.7C, then charged at a constant voltage until the current dropped to 0.02C, and then discharged at a constant current of 1C to 1.5V. This cycle was repeated for 400 cycles, and the discharge capacity of the first cycle and the discharge capacity of the 400th cycle were recorded.

[0112] Calculate the capacity retention during high-temperature cycling using the following formula:

[0113] Capacity retention rate = (Discharge capacity in week 400 / Discharge capacity in week 1) × 100%.

[0114] The specific test results are shown in Table 3.

[0115] Table 2. Corrosion Analysis of Aluminum Current Collectors in Example 1 and Comparative Example 3

[0116]

[0117] As shown in Table 2, the NaFSI content in the non-aqueous electrolyte of Comparative Example 3 is greater than 5%. After 200 cycles of charge and discharge, the corrosion area of ​​the current collector is much larger than that of Example 1. This indicates that the NaFSI content in the non-aqueous electrolyte is between 1% and 5%, which can effectively inhibit the corrosion of the aluminum current collector and provide support for improving the long-term cycle performance of the battery.

[0118] Table 3 Electrical performance test data of Examples 1-28 and Comparative Examples 1-12

[0119]

[0120]

[0121]

[0122] As shown in Tables 1-3, comparing Examples 1, 3, 14, and 15 with Comparative Examples 4 and 5, the conductive agent content in the negative electrode active material layer is between 1% and 10%, resulting in higher rate performance, first-time efficiency, and cycle performance. In Comparative Example 4, the conductive agent content in the negative electrode active material layer is less than 1%, leading to poor rate performance. It is speculated that the low conductive agent content reduces the conductivity of the negative electrode, affecting the high-rate charge and discharge of the battery. In Comparative Example 5, the conductive agent content is higher than 10%, resulting in reduced first-time efficiency. It is speculated that the high conductive agent content reduces the content of the negative electrode active material, affecting the battery's capacity utilization. Comparing Examples 1, 16-20 and Comparative Examples 1 and 8, the battery exhibits higher rate performance, first-time efficiency, and cycle capacity retention when the binder content in the negative electrode active material layer is between 2% and 10%. A binder content between 3% and 6% results in even higher cycle capacity retention and high-rate discharge capacity. In Comparative Example 1, the binder content in the negative electrode active material layer is below 2%, leading to poor cycle performance. It is speculated that the low binder content reduces the adhesion between the negative electrode active material layer and the current collector, thus decreasing the battery's cycle performance. In Comparative Example 8, the binder content is above 10%, resulting in a slight improvement in cycle performance and rate performance, but a decrease in the battery's first-time efficiency. It is speculated that the excessively high binder content reduces the content of the negative electrode active material, affecting the battery's capacity utilization. Comparing Examples 1, 4-5, 8-9 with Comparative Examples 9 and 10, the negative electrode active material with a particle size (c) in the range of 2–12 μm, especially in the range of 3–9 μm, exhibits higher rate performance and cycle performance. This indicates that excessively low particle size (c) of the negative electrode active material affects its dispersibility, reducing the battery's initial efficiency and rate performance. A particle size greater than 12 μm reduces the battery's cycle performance, initial efficiency, and rate performance. Comparing Examples 1, 2, 11-13 with Comparative Examples 2 and 3, the electrolyte with a sodium difluorosulfonamide content between 1% and 5% exhibits higher cycle performance and rate performance, with a preferred range of 1.5%–4%. A sodium difluorosulfonamide content below 1% results in excessively low electrolyte conductivity and low rate performance. A sodium difluorosulfonamide content above 5% leads to poor battery cycle performance, suggesting that a high sodium difluorosulfonamide content easily corrodes the current collector, reducing battery performance. Compared with Comparative Examples 6 and 7, Examples 1, 6-7, 10, and 26-28 show higher cycle performance and rate performance when the propylene carbonate content is in the range of 5% to 40%, with a preferred range of 10% to 30%. When the propylene carbonate content is outside the range of 5% to 40%, the battery's rate performance, initial efficiency, and cycle performance are low. It is speculated that when the propylene carbonate content is outside the range of 5% to 40%, it cannot synergize with sodium bis(fluorosulfonyl)imide, resulting in poor stability of the SEI film formed at the negative electrode interface, thus reducing the battery's initial efficiency, high-rate performance, and cycle performance.

[0123] Comparing Examples 18-25 with Comparative Examples 11-12, the batteries satisfy the following relationship: The battery exhibits high high-rate discharge capacity, initial efficiency, and cycle capacity retention. It is speculated that the binder, particle size, and conductive agent content in the negative electrode active material layer, as well as the sodium bis(fluorosulfonyl)imide and propylene carbonate content in the electrolyte, fall within the range specified in this application, and the battery satisfies the following relationship: The sodium-ion secondary battery prepared with a pH ≤3.3 exhibits a high transport rate at the negative electrode and high conductivity in the electrolyte, enabling high-rate discharge above 5C. Furthermore, the good quality of the SEI film at the negative electrode interface, high electrolyte stability, and high conductivity of the negative electrode all contribute to improved cycle performance and high-rate performance. The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A sodium-ion secondary battery, characterized by, The sodium-ion secondary battery comprises a positive electrode, a negative electrode and a non-aqueous electrolyte, the negative electrode comprises a negative electrode active material layer, the negative electrode active material layer comprises a conductive agent, a binder and a negative electrode active material, the mass content of the conductive agent is a%, the mass content of the binder is b%, and the particle size of the negative electrode active material is c%, based on 100% of the mass of the negative electrode active material layer; The non-aqueous electrolyte comprises an additive and a non-aqueous organic solvent, the additive comprises sodium bisfluorosulfonylimide, and the non-aqueous organic solvent comprises propylene carbonate; the mass content of the sodium bisfluorosulfonylimide is x%, and the mass content of the propylene carbonate is y%, based on 100% of the total mass of the non-aqueous electrolyte; The sodium-ion secondary battery satisfies the following relationship: 0.01≤ ≤3.3； Wherein, 4%≤a%≤6%, 4%≤b%≤6%, c=5μm, 1%≤x%≤2%, and y%=10% or y%=16%.

2. The sodium-ion secondary battery according to claim 1, characterized in that, The sodium-ion secondary battery satisfies the following relationship: 0.02≤ ≤0.8。 3. The sodium-ion secondary battery according to claim 1, characterized in that, The non-aqueous organic solvent further comprises one or more of a carbonate solvent with a carbon atom number of 3-5, a carboxylic acid ester solvent with a carbon atom number of 2-6, and an ether solvent with a carbon atom number of 4-10.

4. The sodium-ion secondary battery according to claim 1, characterized in that, The additive further comprises one or more of a sulfate compound and a fluorinated carbonate compound; The mass content of the sulfate compound is 1%-3%, and the mass content of the fluorinated carbonate compound is 1%-5%, based on 100% of the total mass of the non-aqueous electrolyte.

5. The sodium-ion secondary battery according to claim 1, characterized in that, The non-aqueous electrolyte further comprises a sodium salt, and the sodium salt comprises one or more of sodium perchlorate, sodium tetrafluoroborate, sodium hexafluorophosphate, sodium trifluoroacetate, sodium tetraphenylborate, sodium trifluoromethylsulfonate, sodium bis(fluorosulfonyl)imide and sodium bis(trifluoromethylsulfonyl)imide; The mass content of the sodium salt is 8%-15%, based on 100% of the total mass of the non-aqueous electrolyte.

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