A sodium-ion battery

Abstract

The present invention discloses a sodium-ion battery, comprising: a positive electrode, a negative electrode, and a non-aqueous electrolyte. The non-aqueous electrolyte includes a first additive and a second additive. The first additive includes at least one of sodium bis(oxalato)borate or sodium difluoro(oxalato)borate. The second additive includes at least one of 1,3-propane sultone or ethylene sulfate. The mass content of the first additive in the non-aqueous electrolyte is 0.1 to 3 wt%, and the mass content of the second additive in the non-aqueous electrolyte is 1 to 4 wt%. The median particle size D 50 of the positive electrode active material in the positive electrode is 2 to 12 μm. By performing cyclic voltammetry tests, the current difference between the oxidation peak and the reduction peak of the positive electrode is 0.001 to 0.01 A / g. The sodium-ion battery of the present invention effectively improves the initial efficiency of the sodium-ion battery and improves the cycle performance of the battery by controlling the median particle size of the positive electrode material and defining the film-forming additives in the electrolyte.

Description

Technical Field

[0001] The present invention belongs to the technical field of energy storage devices, and particularly relates to a sodium-ion battery. Background Art

[0002] Sodium-ion batteries have the characteristics of wide resources, low cost and small fluctuations, and have the performance of a wide temperature range and high safety. These characteristics endow sodium-ion batteries with the potential to replace lithium-ion batteries. With the continuous progress of sodium-ion battery technology, sodium-ion batteries occupy an important position in China's energy system, especially in the energy storage field, there is broad room for growth. Therefore, the development of high-performance and low-cost sodium-ion batteries is a decisive factor in determining whether they can be industrialized.

[0003] During the first charge and discharge process of sodium-ion batteries, the electrode material reacts with the electrolyte at the solid-liquid interface to form a passivation layer covering the surface of the electrode material. This passivation layer is an interfacial layer with the characteristics of a solid electrolyte, and this passivation film is called a solid electrolyte interface film, abbreviated as SEI film. The performance of sodium-ion batteries is closely related to the quality of the SEI film. At present, sodium-ion batteries generally have problems such as low first efficiency and insufficient cycling performance due to the low quality of film formation. Therefore, it is necessary to study and improve sodium-ion batteries to improve the film formation quality of the SEI film in order to improve the performance of sodium-ion batteries. Summary of the Invention

[0004] The present invention is made based on the inventor's discovery and recognition of the following facts and problems: Currently, sodium-ion batteries generally have problems of low first efficiency and insufficient cycling performance. Therefore, it is necessary to conduct in-depth research on sodium-ion batteries to improve the first efficiency and cycling performance of sodium-ion batteries.

[0005] The present invention aims to solve at least one of the technical problems in the related art to some extent. For this purpose, the present invention provides a sodium-ion battery, including a positive electrode, a negative electrode and a non-aqueous electrolyte, wherein,

[0006] The non-aqueous electrolyte includes a solvent, an electrolyte salt and an additive. The additive includes a first additive and a second additive. The first additive includes at least one of sodium bis(oxalato)borate (NaBOB) or sodium difluoro(oxalato)borate (NaODFB). The second additive includes at least one of 1,3-propane sultone (RPS) or vinylene sulfate (DTD). The mass content of the first additive in the non-aqueous electrolyte is 0.1-3 wt%, and the mass content of the second additive in the non-aqueous electrolyte is 1-4 wt%;

[0007] The positive electrode includes a positive electrode active material, and the median particle size D 50 of the positive electrode active material is 2-12 μm;

[0008] Cyclic voltammetry (CV) tests were carried out using an electrochemical workstation. The test voltage was 1.5 - 4.0 V, the scanning rate was 0.1 - 1 mV / s, and the current difference corresponding to the oxidation peak and reduction peak of the positive electrode was 0.001 - 0.01 A / g.

[0009] Optionally, the mass content of the first additive in the non-aqueous electrolyte is 0.2 - 2 wt%.

[0010] Optionally, the mass content of the second additive in the non-aqueous electrolyte is 2 - 3 wt%.

[0011] Optionally, the median particle size D of the positive electrode active material 50 is 3 - 9 μm.

[0012] Optionally, the positive electrode active material is selected from at least one of layered metal oxides, polyanion compounds, Prussian compounds, phosphate compounds, and sulfate compounds.

[0013] Optionally, the chemical formula of the layered metal oxide is Na x M y O z , 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; preferably, the layered 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) or at least one of them;

[0014] The chemical formula of the Prussian compound is Na x M[M′(CN)6] y ·zH2O, where M and M′ are transition metals, 0 < x ≤ 2, 0 < y ≤ 1, 0 < z ≤ 20; preferably, the Prussian compound is Na x Mn[Fe(CN)6] y ·nH2O (0 < x ≤ 2, 0 < y ≤ 1, 0 < z ≤ 20) or Na x Fe[Fe(CN)6] y ·nH2O (0 < x ≤ 2, 0 < y ≤ 1, 0 < z ≤ 20) or at least one of them;

[0015] The chemical formula of the phosphate compound is Na3(MO 1-x PO4)2F1+2x where 0 ≤ x ≤ 1, M is selected from at least one of Al, V, Ge, Fe, and Ga. Preferably, the phosphate compound is at least one of Na3(VPO4)2F3 or Na3(VOPO4)2F; or the chemical formula of the phosphate compound is Na2MPO4F, where M is selected from at least one of Fe and Mn. Preferably, the phosphate compound is at least one of Na2FePO4F or Na2MnPO4F;

[0016] The chemical formula of the sulfate compound is Na2M(SO4)2·2H2O, where M is selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V.

[0017] Optionally, the solvent includes at least one of C3 - C8 carbonates, C2 - C6 carboxylates, and C4 - C10 ethers, and the mass content of the solvent in the non-aqueous electrolyte is 70% - 92%;

[0018] Preferably, the carbonate is selected from at least one of ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC), and the mass content of the cyclic carbonate in the non-aqueous electrolyte is not less than 30%.

[0019] Optionally, the electrolyte salt is selected from at least one of sodium perchlorate (NaClO4), sodium tetrafluoroborate (NaBF4), sodium hexafluorophosphate (NaPF6), sodium trifluoroacetate (CF3COONa), sodium tetraphenylborate (NaB(C6H5)4), sodium trifluoromethanesulfonate (NaSO3CF3), sodium bis(fluorosulfonyl)imide (Na[(FSO2)2N]), or sodium bis(trifluoromethylsulfonyl)imide (Na[(CF3SO2)2N]).

[0020] Optionally, the additive further includes fluorinated carbonate. Preferably, the fluorinated carbonate is at least one of fluoroethylene carbonate (FEC) or difluoroethylene carbonate (DFEC).

[0021] Optionally, the negative electrode includes a negative electrode active material, and the negative electrode active material is selected from at least one of hard carbon and soft carbon.

[0022] According to the sodium-ion battery provided by the present invention, in a non-aqueous electrolyte, sodium bis(oxalato)borate (NaBOB) and / or sodium difluoro(oxalato)borate (NaODFB) are added as the first additive, and 1,3-propane sultone (RPS) and / or ethylene sulfate (DTD) are added as the second additive. The two can decompose on the surface of the positive electrode during the battery formation stage to form an interfacial film, which can effectively maintain the structural stability of the positive electrode active material. Through a large number of studies, the inventor found that by controlling the content of the first additive, the content of the second additive, and the median particle size of the positive electrode active material, it is beneficial to improve the film-forming quality of the interfacial film. The film-forming quality of the interfacial film will affect the magnitude of the current difference corresponding to the oxidation peak and the reduction peak of the positive electrode in the CV test, thereby affecting the initial efficiency and cycling performance of the battery. The inventor found that controlling the current difference corresponding to the oxidation peak and the reduction peak of the positive electrode in the CV test within the range of 0.001 to 0.01 A / g can effectively improve the performance of the battery. It is speculated that the median particle size of the positive electrode active material will affect the wetting degree of the non-aqueous electrolyte on the positive electrode and the shuttling of sodium ions in the positive electrode. When the content of the first additive is 0.1 to 3 wt%, the content of the second additive is 1 to 4 wt%, and the median particle size D 50 is 2 to 12 μm, the three can achieve a good cooperation effect, effectively improving the cycling performance and initial efficiency of the sodium-ion battery at high temperatures. Detailed Embodiments

[0023] The following details the embodiments of the present invention. The embodiments are exemplary and are intended to explain the present invention, but should not be construed as limiting the present invention. A sodium-ion battery according to an embodiment of the present invention includes: a positive electrode, a negative electrode, and a non-aqueous electrolyte. Among them,

[0024] The non-aqueous electrolyte includes a solvent, an electrolyte salt, and an additive. The additive includes a first additive and a second additive. The first additive includes at least one of sodium bis(oxalato)borate (NaBOB) or sodium difluoro(oxalato)borate (NaODFB). The second additive includes at least one of 1,3-propane sultone (RPS) or ethylene sulfate (DTD). The mass content of the first additive in the non-aqueous electrolyte is 0.1 to 3 wt%, and the mass content of the second additive in the non-aqueous electrolyte is 1 to 4 wt%;

[0025] The positive electrode includes a positive electrode active material, and the median particle size D 50 of the positive electrode active material is 2 to 12 μm;

[0026] A cyclic voltammetry test is performed using an electrochemical workstation. The test voltage is 1.5 to 4.0 V, and the scanning rate is 0.1 to 1 mV / s. The current difference corresponding to the oxidation peak and the reduction peak of the positive electrode is 0.001 to 0.01 A / g.

[0027] The first additive and the second additive will form a stable interface film at the positive electrode interface. The film-forming quality of this interface film will affect the magnitude of the current difference corresponding to the oxidation peak and the reduction peak in the CV test of the positive electrode, thereby affecting the initial efficiency and cycle performance of the battery. The film-forming quality of this interface film is mainly related to the median particle size D of the positive electrode active material 50 size, the mass contents of the first additive and the second additive, and the film-forming quality of the interface film can also be finely adjusted through different formation conditions. During the formation process, for batteries at different formation potentials, there are slightly differences in the quality of the interface films formed. Moreover, as the formation potential increases, the growth of the film layer tends to be complete. At the same time, the formation current, formation time, formation temperature, etc. will also affect the quality of the interface film.

[0028] In some embodiments, the formation conditions of the sodium-ion battery include the following operating steps:

[0029] Perform constant-current charging formation on the sodium-ion battery. Charge the sodium-ion battery at a rate of 0.03 - 0.1C for 2 - 4h, then charge at a constant current of 0.1 - 0.3C until it stops charging when reaching 3.0 - 3.8V. Then age the sodium-ion battery at 30 - 50°C for 5 - 30min and perform secondary vacuum sealing; continue to charge and form the sodium-ion battery at a rate of 0.1 - 0.3C. When the formation potential reaches 3.5 - 4.0V, stop charging. Then age the sodium-ion battery at 30 - 50°C for 10 - 48h, and continue to charge and form the lithium-ion battery at a rate of 0.3 - 0.5C until reaching 100% SOC to form a specific interface film on the surface of the positive electrode, so that the current difference corresponding to the oxidation peak and the reduction peak of the positive electrode is 0.001 - 0.01A / g.

[0030] In a specific embodiment, the mass content of the first additive in the non-aqueous electrolyte can be 0.1wt%, 0.2wt%, 0.4wt%, 0.5wt%, 0.7wt%, 0.9wt%, 1.0wt%, 1.1wt%, 1.3wt%, 1.5wt%, 1.8wt%, 2.0wt%, 2.3wt%, 2.7wt%, 3.0wt%.

[0031] In a preferred embodiment, the mass content of the first additive in the non-aqueous electrolyte is 0.2 - 2wt%.

[0032] The first additive and the second additive have a synergistic effect in improving the quality of the formed interface film. When the mass content of the first additive in the non-aqueous electrolyte is too low, it is difficult to play a synergistic role with the second additive, affecting the film-forming quality of the interface film on the positive electrode material layer; when the mass content of the first additive in the non-aqueous electrolyte is too high, the formed interface film is too thick, easily leading to an increase in the impedance of the sodium-ion battery, which is not conducive to the improvement of the cycle performance of the sodium-ion battery.

[0033] In a specific embodiment, the mass content of the second additive in the non-aqueous electrolyte may be 1.0 wt%, 1.1 wt%, 1.3 wt%, 1.5 wt%, 1.8 wt%, 2.0 wt%, 2.3 wt%, 2.7 wt%, 3.0 wt%, 3.1 wt%, 3.2 wt%, 3.3 wt%, 3.5 wt%, 3.7 wt%, 3.9 wt% or 4.0 wt%.

[0034] In a preferred embodiment, the mass content of the second additive in the non-aqueous electrolyte is 2-3 wt%.

[0035] If the mass content of the second additive in the non-aqueous electrolyte is too low, it is difficult to play a synergistic role with the first additive, which will exacerbate the loss of irreversible capacity in the first cycle, deteriorate the initial efficiency of the battery, and also have limited improvement on the high-temperature cycle performance of the sodium-ion battery; when the mass content of the second additive in the non-aqueous electrolyte is too high, it will lead to too high a thickness of the interfacial film on the surface of the positive electrode, thereby affecting the insertion and extraction of sodium ions in the positive electrode, affecting the ion conduction efficiency, and deteriorating the cycle performance and initial efficiency.

[0036] In a preferred embodiment, the mass content of the second additive in the non-aqueous electrolyte is greater than that of the first additive.

[0037] The second additive will preferentially participate in film formation before the first additive during the activation stage of the sodium-ion battery. Preferably, the addition amount of the second additive is greater than that of the first additive, which is beneficial to the effective film formation of the second additive and the formation of an excellent and stable electrode interfacial film in synergy with the first additive, further reducing the consumption of the electrolyte and the irreversible capacity loss in the first cycle, being beneficial to improving the reversibility of the battery during the cycle process, and further enhancing the cycle performance and initial efficiency of the battery.

[0038] In a specific embodiment, the median particle size D of the positive electrode active material 50 may be 2 μm, 3 μm, 3.5 μm, 4 μm, 4.6 μm, 5 μm, 5.3 μm, 6 μm, 6.8 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm or 12 μm. In a preferred embodiment, the median particle size D of the positive electrode active material 50 is 3-9 μm.

[0039] The insertion and extraction reactions of sodium ions mainly occur at the interface between the positive electrode and the non-aqueous electrolyte. If the median particle size of the positive electrode active material is too small and the specific surface area is large, it will excessively consume the electrolyte to form a film, resulting in too low initial efficiency. Moreover, if the particle size is too small, the wettability of the electrode will deteriorate, degrading the rate performance. If the median particle size of the positive electrode active material is too large, it will extend the diffusion path of sodium ions in the material, leading to a slow charge transfer rate and a decline in rate performance.

[0040] In some embodiments, the positive electrode active material is selected from at least one of layered metal oxides, polyanion compounds, Prussian compounds, phosphate compounds, and sulfate compounds.

[0041] In some preferred embodiments, the chemical formula of the layered metal oxide is Na x M y O z , 0 < x ≤ 1, 0 < y ≤ 1, 1 < z ≤ 2, M is selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, V. More preferably, the layered 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) or at least one of them;

[0042] The chemical formula of the Prussian compound is Na x M[M′(CN)6] y ·zH2O, M and M′ are transition metals, 0 < x ≤ 2, 0 < y ≤ 1, 0 < z ≤ 20; preferably, the Prussian compound is Na x Mn[Fe(CN)6] y ·nH2O (0 < x ≤ 2, 0 < y ≤ 1, 0 < z ≤ 20) or Na x Fe[Fe(CN)6] y ·nH2O (0 < x ≤ 2, 0 < y ≤ 1, 0 < z ≤ 20) or at least one of them;

[0043] The chemical formula of the phosphate compound is Na3(MO 1-x PO4)2F 1+2x, where 0 ≤ x ≤ 1, M is selected from at least one of Al, V, Ge, Fe, and Ga. Preferably, the phosphate compound is at least one of Na3(VPO4)2F3 or Na3(VOPO4)2F; or, the chemical formula of the phosphate compound is Na2MPO4F, and M is selected from at least one of Fe and Mn. Preferably, the phosphate compound is at least one of Na2FePO4F or Na2MnPO4F;

[0044] The chemical formula of the sulfate compound is Na2M(SO4)2·2H2O, and M is selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, and V.

[0045] In the embodiments of the present invention, the cathode active material in the cathode is preferably selected, especially the ternary layered oxide NaNi m Fe n Mn p O2, which can form a film synergistically with the additives used in the present invention, improve the film-forming quality, and is beneficial to further improving the performance of the battery.

[0046] In some embodiments, the solvent in the non-aqueous electrolyte includes at least one of C3-C8 carbonates, C2-C6 carboxylates, and C4-C10 ethers, and the mass content of the solvent in the non-aqueous electrolyte is 70% to 92%.

[0047] In a preferred embodiment, the C3-C8 carbonates include cyclic carbonates or chain carbonates with 3 to 5 carbon atoms. Further preferably, the cyclic carbonates include at least one of ethylene carbonate, vinylene carbonate, ethylene ethylenecarbonate, propylene carbonate, γ-butyrolactone, and butylene carbonate; the chain carbonates include at least one of dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and dipropyl carbonate; more preferably, the carbonate is selected from at least one of ethylene carbonate (EC), propylene carbonate (PC), and ethyl methyl carbonate (EMC), and the mass content of the cyclic carbonate in the non-aqueous electrolyte is not less than 30%.

[0048] In a preferred embodiment, the C2-C6 carboxylates include at least one of methyl acetate, ethyl acetate, propyl acetate, butyl acetate, and propyl propionate.

[0049] In a preferred embodiment, the C4-C10 ethers include cyclic ethers or chain ethers with 4 to 10 carbon atoms; further preferably, the cyclic ethers include at least one of 1,3-dioxolane, 1,4-dioxane, tetrahydrofuran, 2-methyltetrahydrofuran, and 2-trifluoromethyltetrahydrofuran; the chain ethers include at least one of dimethoxymethane, 1,2-dimethoxyethane, and diethylene glycol dimethyl ether.

[0050] In the embodiments of the present invention, there is no particular limitation on the solvent, and the solvents commonly used in sodium-ion batteries can all be applicable to the present invention.

[0051] In some embodiments, the electrolyte salt in the non-aqueous electrolyte is selected from at least one of sodium perchlorate (NaClO 4) , sodium tetrafluoroborate (NaBF4), sodium hexafluorophosphate (NaPF6), sodium trifluoroacetate (CF3COONa), sodium tetraphenylborate (NaB(C6H5)4), sodium trifluoromethanesulfonate (NaSO3CF3), sodium bis(fluorosulfonyl)imide (Na[(FSO2)2N]) or sodium bis(trifluoromethylsulfonyl)imide (Na[(CF3SO2)2N]). There is no particular limitation on the electrolyte salt in the embodiments of the present invention. It is preferably to use the selected electrolyte salt, especially sodium hexafluorophosphate (NaPF6), sodium trifluoromethanesulfonate (NaSO3CF3) or sodium bis(fluorosulfonyl)imide (Na[(FSO2)2N]), which can make the battery exhibit the best comprehensive performance.

[0052] In some embodiments, the additive further includes fluorinated carbonate. Preferably, the fluorinated carbonate is at least one of fluoroethylene carbonate (FEC) or difluoroethylene carbonate (DFEC). In the embodiments of the present invention, the electrolyte can further include the additive fluorinated carbonate, which can further improve the film-forming quality and is beneficial to improving the battery performance.

[0053] In some embodiments, the negative electrode includes a negative electrode active material, and the negative electrode active material is selected from at least one of hard carbon and soft carbon. In the embodiments of the present invention, there is no particular limitation on the negative electrode active material, and the negative electrode materials that can be used in sodium-ion batteries in the prior art can all be applicable to the present invention.

[0054] The present invention will be described in detail below with reference to embodiments.

[0055] Example 1

[0056] (1) Preparation of the electrolyte: 6.5 wt% of the electrolyte salt NaPF6, 89 wt% of the solvent (ethylene carbonate EC: propylene carbonate PC: ethyl methyl carbonate EMC = 1:1:1), 0.5 wt% of the first additive sodium bis(oxalato)borate (NaBOB), and 4 wt% of the second additive 1,3-propane sultone (RPS) were mixed uniformly to obtain the electrolyte.

[0057] (2) Preparation of the positive electrode: The binder polyvinylidene fluoride was dispersed, the solvent N-methylpyrrolidone, the conductive agent acetylene black, and the positive electrode active material NaNi with a median particle size D 50 of 5.3 μm 1 / 3 Fe 1 / 3 Mn 1 / 3Mix O2 evenly to obtain the positive electrode paste. Coat the positive electrode paste on the current collector, and obtain the positive electrode sheet through the processes of coating, drying, and rolling.

[0058] (3) Preparation of the negative electrode: Mix the negative electrode active material hard carbon, the conductive agent carbon black, the solvent water, and the binder sodium carboxymethyl cellulose evenly to obtain the negative electrode paste. Coat the negative electrode paste on the current collector, and obtain the negative electrode sheet through the processes of coating, drying, and rolling.

[0059] (4) Battery assembly

[0060] Place a separator between the above-prepared positive electrode sheet and negative electrode sheet, then wind the sandwich structure composed of the positive electrode sheet, negative electrode sheet, and separator, and then flatten the wound body and put it into an aluminum foil packaging bag. Bake it in vacuum at 75 °C for 48 h to obtain the battery cell to be filled with electrolyte; Inject the above-prepared non-aqueous electrolyte into the battery cell through the injection hole, and the amount of the electrolyte should ensure that the voids in the battery cell are filled.

[0061] Then carry out the film-forming process of constant current charging and formation according to the following steps: Charge the sodium-ion battery at a constant current of 0.05C for 180 min, then charge it at a constant current of 0.3C until it stops charging when the voltage reaches 3.5V, then age the sodium-ion battery at room temperature for 30 min and perform secondary vacuum sealing; Continue to charge and form the sodium-ion battery at a rate of 0.3C. When the formation potential reaches 3.95V, stop charging, then age the sodium-ion battery at room temperature for 24 h, and continue to charge and form the sodium-ion battery at a rate of 0.3C until 100% SOC is reached to obtain the sodium-ion battery.

[0062] Use an electrochemical workstation of model VSP-300 to perform cyclic voltammetry (CV) tests on the positive electrode of the sodium-ion battery. The test voltage range is 1.5 - 4.0V, the scanning rate is 0.1 mV / s, and the test results are shown in Table 1.

[0063] Examples 2 - 20

[0064] Examples 2 - 20 are used to illustrate the sodium-ion battery and its preparation method disclosed in the present invention, including most of the operation steps in Example 1. The differences are as follows:

[0065] The median particle size D of the positive electrode active material used in Examples 2 - 20 50 And the types and mass contents of each additive in the non-aqueous electrolyte are shown in Table 1.

[0066] By using a specific particle size D 50On the basis of a non-aqueous electrolyte containing a positive electrode active material and adding a first additive and a second additive in a specified ratio, a charging formation treatment is carried out under specified film-forming conditions, so as to form a specific interfacial film on the surface of the positive electrode, such that the current difference corresponding to the oxidation peak and the reduction peak of the positive electrode is 0.001 to 0.01 A / g, as shown in Table 1.

[0067] Comparative Examples 1 to 9

[0068] Comparative Examples 1 to 9 are used to comparatively illustrate the sodium-ion battery and its preparation method disclosed in the present invention, including most of the operation steps in Example 1. The differences are as follows:

[0069] The median particle size D of the positive electrode active material used in Comparative Examples 1 to 9 50 and the types and mass contents of the respective additives in the non-aqueous electrolyte are as shown in Table 1.

[0070] Performance test

[0071] The following performance tests are carried out on the sodium-ion battery prepared above:

[0072] (1) Initial activation first efficiency of the battery

[0073] Measure the total capacity C1 of the sodium-ion battery after the film-forming process is completed, and then measure the capacity C2 released when the sodium-ion battery is discharged from 3.95 V to 1.5 V at a rate of 0.2C.

[0074] Initial activation first efficiency = C2 / C1 * 100%

[0075] (2) Discharge capacity ratio at 4C rate

[0076] Measure the capacity C3 released when the sodium-ion battery is discharged from 3.95 V to 1.5 V at a rate of 4C and the capacity C2 released when the battery is discharged from 3.95 V to 1.5 V at a rate of 0.2C during the initial activation stage. The calculation formula is as follows:

[0077] Discharge capacity ratio at 4C rate = C3 / C2.

[0078] (3) Cycle performance test

[0079] Place the sodium-ion battery in a constant temperature environment of 25°C, charge it at a constant current of 0.5C to 3.95 V, then charge it at a constant voltage until the current drops to 0.03C, and then discharge it at a constant current of 1C to 1.5 V. Repeat this cycle, and record the discharge capacity of the first cycle and the discharge capacity of the last cycle.

[0080] Calculate the capacity retention rate of cycling at 25°C according to the following formula:

[0081] Capacity retention rate (%) = Discharge capacity of the last cycle / Discharge capacity of the first cycle × 100%.

[0082] Place the sodium-ion battery in a constant-temperature environment at 45°C, charge it at a constant current of 0.5C to 3.95V, then charge it at a constant voltage until the current drops to 0.03C, and then discharge it at a constant current of 1C to 1.5V. Repeat this cycle, and record the discharge capacity and battery volume of the first cycle, as well as the discharge capacity and battery volume of the last cycle.

[0083] Calculate the capacity retention rate and gas expansion rate of the cycle at 45°C according to the following formula:

[0084] Capacity retention rate (%) = Discharge capacity of the last cycle / Discharge capacity of the first cycle × 100%;

[0085] Gas expansion rate (%) = Battery volume of the last cycle / Battery volume of the first cycle × 100%.

[0086] The battery parameters and electrical performance data of Examples 1-20 and Comparative Examples 1-9 are shown in Tables 1 and 2.

[0087] Table 1

[0088]

[0089] Table 2

[0090]

[0091] It can be seen from Tables 1 and 2 that for the sodium-ion batteries prepared in Examples 1-20, using the electrolytes composed of the first additive and the second additive in the designed dosages of the present invention, and using the cathode active material with the selected particle size of the present invention, after formation, the current difference between the oxidation peak and the reduction peak of the CV curve corresponding to the cathode can meet the range requirements of 0.001-0.01A / g, improving the cycle reversibility. At the same time, the second additive can preferentially participate in film formation during the battery activation stage, effectively reducing the consumption of the electrolyte and the irreversible capacity loss in the first cycle. It synergistically acts with the first additive to form a stable electrode interface film, and the battery can maintain good reversibility during the cycle, achieving the purpose of improving the cycle and first efficiency of the full battery.

[0092] In Comparative Examples 1 and 2, only one additive was used, and the purpose of the synergistic effect of the two additives could not be achieved. The current differences between the oxidation peak and the reduction peak reached 0.0280A / g and 0.0125A / g respectively, far exceeding the requirement of not exceeding 0.01A / g that the present invention hopes to control, indicating poor film formation quality and poor stability of the formed interface film during the cycle. It is necessary to continuously consume the electrolyte to participate in the repair of the interface film, resulting in extremely unstable interfaces at both electrodes and intense side reactions, thus deteriorating the battery performance.

[0093] In Comparative Example 3, too much of the first additive was added, and it excessively participated in film formation, resulting in uneven film formation of the interfacial film. The current difference between the oxidation peak and the reduction peak reached 0.0118 A / g, deteriorating the battery cycle performance, especially the high-temperature cycle performance. After 500 cycles at 45°C, the capacity retention rate was only 68.6%, and the gas expansion rate during cycling at 45°C was as high as 42.7%. In Comparative Example 4, too much of the second additive was added, and it could not effectively synergize with the first additive, resulting in an overly thick interfacial film on the positive electrode surface, poor film formation quality of the SEI film, affecting the insertion and extraction of sodium ions in the positive electrode, reducing the ion conduction efficiency, and thus deteriorating the cycle performance and the initial efficiency, causing the initial efficiency to drop to 78.6% and the capacity retention rate after 500 cycles at 45°C to be only 76.1%.

[0094] In Comparative Example 5, the particle size of the positive electrode active material used was too small, and the specific surface area of the active substance was relatively large, which would excessively consume the electrolyte to participate in film formation, resulting in too low initial efficiency, only 65.2%. Moreover, the wettability of the electrode was poor, deteriorating the rate performance. The discharge capacity ratio at 4C rate was only 80.9%. At the same time, the cycle performance was also reduced. After 500 cycles at 45°C, the capacity retention rate was only 70.2%, and the gas expansion rate during cycling at 45°C was as high as 46.3%. In Comparative Example 6, the particle size of the positive electrode active material used was too large, extending the diffusion path of sodium ions in the material, resulting in a decrease in the sodium ion transmission rate, deteriorating the battery rate performance. The discharge capacity ratio at 4C rate was only 68.8%. At the same time, the cycle performance was also reduced. After 500 cycles at 45°C, the capacity retention rate was only 74.3%, and the gas expansion rate during cycling at 45°C was as high as 43.2%.

[0095] In Comparative Example 7, too little of the second additive was added and it could not effectively participate in film formation, resulting in poor and unstable film formation on the electrode, prone to side reactions. The current difference between the oxidation peak and the reduction peak reached 0.0095 A / g, causing the initial efficiency, rate performance, and cycle performance to all significantly decline compared with the present invention. In Comparative Example 8, too little of the first additive was added and it could not effectively participate in film formation, resulting in poor cycle reversibility of the battery. After 500 cycles at 45°C, the capacity retention rate was only 73.8%, and the gas expansion rate during cycling at 45°C was as high as 40.6%. At the same time, the formed film was uneven and had poor stability, deteriorating the battery performance.

[0096] In Comparative Example 9, the addition amounts of both the first additive and the second additive were excessive, and the additives excessively participated in film formation, resulting in a too thick and uneven thickness of the interfacial film, poor film formation quality, an increase in the impedance of the sodium-ion battery, a significant decrease in the ion transport efficiency, and serious deterioration of the battery performance. In Comparative Example 9, due to the poor film formation quality, the current difference corresponding to the oxidation peak and the reduction peak was as high as 0.0123 A / g, the battery performance was significantly decreased, the initial efficiency of the battery was only 68.4%, the discharge capacity ratio at 4C rate was only 68.2%, the capacity retention rate after 500 cycles at 25°C was only 65.3%, the capacity retention rate after 500 cycles at 45°C was only 63.2%, and the gas expansion rate during cycling at 45°C was as high as 52.3%.

[0097] In the present invention, the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples", etc. mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0098] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A sodium-ion battery, characterized in that, It includes a positive electrode, a negative electrode and a non-aqueous electrolyte. Among them, the non-aqueous electrolyte includes a solvent, an electrolyte salt and an additive. The additive consists of a first additive and a second additive. The first additive is selected from at least one of sodium bis(oxalato)borate or sodium difluoro(oxalato)borate. The second additive includes at least one of 1,3-propane sultone or ethylene sulfate. The mass content of the first additive in the non-aqueous electrolyte is 0.1~3wt%, and the mass content of the second additive in the non-aqueous electrolyte is 1~4wt%. The mass content of the second additive in the non-aqueous electrolyte is greater than that of the first additive; The positive electrode includes a positive electrode active material, and the median particle size D 50 of the positive electrode active material is 2 to 7 μm; Cyclic voltammetry test is carried out using an electrochemical workstation. The test voltage is 1.5~4.0V, and the scanning rate is 0.1~1mV / s. The current difference corresponding to the oxidation peak and reduction peak of the positive electrode is 0.001~0.01A / g; Among them, the formation conditions of the sodium-ion battery include the following operation steps: The sodium-ion battery is charged and formed under constant current. The sodium-ion battery is charged at a rate of 0.03~0.1C for 2~4h, and then charged at a constant current of 0.1~0.3C until the voltage reaches 3.0~3.8V and then the charging is stopped. Then the sodium-ion battery is aged at 30~50°C for 5~30min and sealed in vacuum; continue to charge and form the sodium-ion battery at a rate of 0.1~0.3C. When the formation potential reaches 3.5~4.0V, the charging is stopped. Then the sodium-ion battery is aged at 30~50°C for 10 - 48h. Continue to charge and form the lithium-ion battery at a rate of 0.3~0.5C until 100% SOC is reached to form an interfacial film on the surface of the positive electrode, so that the current difference corresponding to the oxidation peak and reduction peak of the positive electrode is 0.001~0.01A / g.

2. The sodium-ion battery according to claim 1, characterized in that The mass content of the first additive in the non-aqueous electrolyte is 0.2~2wt%.

3. The sodium ion battery according to claim 1, wherein The mass content of the second additive in the non-aqueous electrolyte is 2~3wt%.

4. The sodium-ion battery according to claim 1, wherein, The median particle size D of the positive electrode active material 50 is 3 to 7 μm.

5. The sodium ion battery according to claim 1, characterized in that, The positive electrode active material is selected from at least one of layered metal oxides, polyanion compounds, Prussian compounds, phosphate compounds, sulfate compounds.

6. The sodium ion battery according to claim 5, wherein The chemical formula of the layered metal oxide is Na x M y O z , where 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 molecular formula of the Prussian compound is Na x M[M′(CN)6] y ·zH2O, where M and M′ are transition metals, 0 < x ≤ 2, 0 < y ≤ 1, 0 < z ≤ 20; The chemical formula of the phosphate compound is Na3(MO 1-x PO4)2F 1+2x , where 0 ≤ x ≤ 1 and M is selected from at least one of Al, V, Ge, Fe, and Ga; alternatively, the chemical formula of the phosphate compound is Na2MPO4F, and M is selected from at least one of Fe and Mn; The chemical formula of the sulfate compound is Na2M(SO4)2·2H2O, and M is selected from at least one of Cr, Fe, Co, Ni, Cu, Mn, Sn, Mo, Sb, V.

7. The sodium ion battery according to claim 6, wherein The layered 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), or at least one of them.

8. The sodium-ion battery according to claim 6, characterized in that, The Prussian compound is Na x Mn[Fe(CN)6] y ·nH2O (0 < x ≤ 2, 0 < y ≤ 1, 0 < z ≤ 20) or Na x Fe[Fe(CN)6] y ·nH2O (0 < x ≤ 2, 0 < y ≤ 1, 0 < z ≤ 20), or at least one of them.

9. The sodium-ion battery according to claim 6, characterized in that, The phosphate compound is at least one of Na3(VPO4)2F3 or Na3(VOPO4)2F.

10. The sodium ion battery according to claim 6, wherein The phosphate compound is at least one of Na2FePO4F or Na2MnPO4F.

11. The sodium ion battery according to claim 1, wherein, The solvent includes at least one of C3~C8 carbonates, C2~C6 carboxylates, C4~C10 ethers.

12. The sodium ion battery according to claim 11, wherein, The carbonate is selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, and the mass content of cyclic carbonate in the non-aqueous electrolyte is not less than 30%.

13. The sodium ion battery according to claim 1, characterized in that, The electrolyte salt is selected from at least one of sodium perchlorate, sodium tetrafluoroborate, sodium hexafluorophosphate, sodium trifluoroacetate, sodium tetraphenylborate, sodium trifluoromethanesulfonate, sodium bis(fluorosulfonyl)imide or sodium bis(trifluoromethylsulfonyl)imide.

14. The sodium-ion battery according to claim 1, wherein, The additive further includes a fluorinated carbonate.

15. The sodium ion battery according to claim 14, wherein, The fluorinated carbonate is at least one of fluoroethylene carbonate or difluoroethylene carbonate.

16. The sodium ion battery according to claim 1, characterized in that The negative electrode includes a negative electrode active material, and the negative electrode active material is selected from at least one of hard carbon or soft carbon.

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