Sodium ion battery positive electrode interface film and preparation method therefor, and sodium ion battery

Abstract

A sodium ion battery positive electrode interface film and a preparation method therefor, and a sodium ion battery. An assembled sodium ion battery is pre-discharged to the reduction potential of an electrolyte for constant voltage discharge, so that the electrolyte is reduced on the surface of a positive electrode to form a positive interface film in situ. The formed positive electrode-electrolyte interface film containing an inorganic matter has a more compact structure, has reduced solubility, can stabilize transition metal ions, alleviates the structure and interface damage of a positive electrode material in the cycle process, and effectively improves the self-discharge performance, cycle performance and coulombic efficiency of the sodium ion battery.

Description

Sodium ion battery positive electrode interface film and preparation method thereof, sodium ion battery Technical Field

[0001] The present application relates to the field of sodium ion batteries. Specifically, the present invention relates to a sodium ion battery positive electrode interface film and a preparation method thereof, and a sodium ion battery. Background Art

[0002] The large-scale development of the energy storage market and the continuous expansion of energy storage application scenarios have placed higher demands on the economic efficiency and safety of energy storage technologies. The high cost of upstream raw materials and the scarcity of lithium resources will hinder the large-scale application of lithium-ion battery energy storage technology. As the chemical energy storage technology closest to lithium-ion batteries, sodium-ion batteries offer advantages such as abundant sodium resources, low cost, good safety, and excellent rate and low-temperature performance, creating significant potential for large-scale application. Therefore, sodium-ion batteries can serve as an important complementary technology to lithium-ion batteries in the field of large-scale energy storage, with significant economic value and strategic significance.

[0003] However, compared to lithium-ion batteries, sodium-ion batteries currently have problems such as low energy density and short cycle life, which largely depends on the performance of the positive electrode material. Because the radius of sodium ions is larger than that of lithium ions, the positive electrode materials, especially layered transition metal oxides, experience more severe volume changes and more complex phase transitions during the process of sodium ion insertion and extraction, and the surface structure stability is worse. In addition, at the same voltage, the sodium positive electrode is at a higher potential and has a higher surface residue, which makes the circulation and gas production problems prominent. Therefore, the surface interface design of the positive electrode of sodium-ion batteries is particularly important.

[0004] A stable cathode-electrolyte interface (CEI) can effectively resist a series of side reactions at the interface during cycling, significantly improving battery performance. Currently, researchers primarily optimize electrolyte composition to improve the oxidation pathways and oxidation products of the electrolyte, thereby improving the structure and composition of the CEI. However, this approach primarily generates CEI through the oxidation or catalytic decomposition of the solvent in the electrolyte. Its primary component is organic matter, resulting in a relatively loose structure that cannot effectively resist a series of side reactions at the interface during cycling. Technical issues

[0005] The interfacial film formed by conventional electrolytes has a relatively simple composition and a certain solubility, which cannot effectively inhibit the reduction of transition metals, the irreversible changes in the surface crystal structure and the continuous oxidative decomposition of the electrolyte, resulting in serious battery degradation. Technical Solutions

[0006] The present invention is based on the inventors' discovery and understanding of the following facts and problems: Currently, researchers mainly improve the oxidation path and oxidation products of the electrolyte by optimizing the electrolyte composition to achieve the purpose of improving the CEI structure and composition. However, under this method, the generation of CEI mainly comes from the oxidation or catalytic decomposition of the solvent in the electrolyte. The main component is organic matter, and the structure is relatively loose, which cannot effectively resist a series of side reactions at the interface during the cycle. The interfacial film formed by conventional electrolytes is relatively simple in composition and has a certain solubility. It cannot effectively inhibit the reduction of transition metals, the irreversible change of the surface crystal structure and the continuous oxidative decomposition of the electrolyte, resulting in serious battery degradation.

[0007] The present invention aims to solve, at least to a certain extent, one of the technical problems in the related art. To this end, embodiments of the present invention provide a sodium ion battery positive electrode interface film, a preparation method thereof, and a sodium ion battery. By pre-discharging the assembled sodium ion battery to the reduction potential of the electrolyte and then performing constant voltage discharge, the electrolyte can be reduced on the positive electrode surface to form a positive electrode-electrolyte interface film containing inorganic substances. The structure is more compact, the solubility of the interface film is reduced, and the transition metal ions can be stabilized, thereby alleviating the structural and interfacial damage of the positive electrode material during the cycle process, and effectively improving the self-discharge performance, cycle performance, and coulombic efficiency of the sodium ion battery.

[0008] An embodiment of the present invention provides a positive electrode interface film for a sodium ion battery. The assembled sodium ion battery is pre-discharged to the reduction potential of the electrolyte and then discharged at a constant voltage, so that the electrolyte is reduced on the positive electrode surface, thereby forming a positive electrode interface film in situ.

[0009] The advantages and technical effects of the sodium-ion battery positive electrode interface film of the embodiment of the present invention are as follows: By pre-discharging the assembled sodium-ion battery to the reduction potential of the electrolyte and then performing constant-voltage discharge, the electrolyte can be reduced on the positive electrode surface, forming a positive electrode-electrolyte interface film containing inorganic substances. Unlike the loose and porous CEI membrane rich in organic matter formed by the initial oxidation of the electrolyte, the interface film of the present invention has a denser structure, reduces the solubility of the interface film, stabilizes transition metal ions, alleviates the structural and interfacial damage of the positive electrode material during the cycle process, and effectively improves the self-discharge performance, cycle performance, and coulombic efficiency of the sodium-ion battery.

[0010] In some embodiments, the sodium ion battery positive electrode interface film includes at least one of a fluorine-containing organic compound, sodium fluoride, a boron-containing inorganic compound, and a phosphorus-containing inorganic compound.

[0011] In some embodiments, the electrolyte includes an additive, wherein the additive includes at least one of a fluorine-containing additive, a boron-containing additive, or a phosphorus-containing additive;

[0012] And / or, the positive electrode comprises a layered transition metal oxide; the layered transition metal oxide comprises NaFe 1 / 3 Ni 1 / 3 Mn 1 / 3 O2、Na 2 / 3 Ni 1 / 3 Mn 2 / 3 O2、NaFe 0.5 Mn 0.5 At least one of O2;

[0013] And / or, the electrolyte comprises a sodium salt and a solvent; the sodium salt comprises at least one of sodium hexafluorophosphate or sodium perchlorate; the concentration of the sodium salt in the electrolyte is 0.8-1.5 M; the solvent comprises a carbonate; the carbonate comprises at least one of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, propylene carbonate, and diethyl carbonate.

[0014] In some embodiments, the fluorine-containing additive includes at least one of fluoroethylene carbonate, bisfluoroethylene carbonate, trifluoropropylene carbonate, and sodium bis(trifluoromethanesulfonyl)imide;

[0015] And / or, the boron-containing additive includes at least one of sodium tetrafluoroborate and sodium difluorooxalatoborate;

[0016] And / or, the phosphorus-containing additive includes at least one of trimethyl phosphate, triethyl phosphate, and tris(2,2,2-trifluoroethyl)phosphate.

[0017] In some embodiments, the additive includes fluoroethylene carbonate; the volume percentage of the fluoroethylene carbonate in the electrolyte is 2-10%;

[0018] And / or, the additives include fluoroethylene carbonate and sodium tetrafluoroborate; the volume percentage of the fluoroethylene carbonate in the electrolyte is 2-10%, and the mass percentage of the sodium tetrafluoroborate in the electrolyte is 0.5-5%;

[0019] And / or, the additives include fluoroethylene carbonate, sodium tetrafluoroborate and triethyl phosphate; the volume percentage of the fluoroethylene carbonate in the electrolyte is 2-10%, the mass percentage of the sodium tetrafluoroborate in the electrolyte is 0.5-5%, and the volume percentage of the triethyl phosphate in the electrolyte is 5-15%.

[0020] An embodiment of the present invention provides a method for preparing a positive electrode interface film of a sodium ion battery, comprising the following steps:

[0021] (1) Assemble the battery by combining the positive electrode, negative electrode and electrolyte;

[0022] (2) The battery is pre-discharged to the reduction potential of the electrolyte, and then discharged at a constant voltage to obtain a positive electrode interface film of the sodium ion battery.

[0023] In an embodiment of the present invention, by pre-discharging the assembled sodium-ion battery to the reduction potential of the electrolyte and then performing constant-voltage discharge, the electrolyte can be reduced on the positive electrode surface, forming a positive electrode-electrolyte interface film containing inorganic substances. Unlike the loose, porous CEI film rich in organic matter formed by the initial oxidation of the electrolyte, the interface film structure of the present invention is more compact, reducing the solubility of the interface film, stabilizing transition metal ions, and alleviating structural and interfacial damage of the positive electrode material during the cycle process, effectively improving the self-discharge performance, cycle performance, and coulombic efficiency of the sodium-ion battery.

[0024] In some embodiments, in step (1), the negative electrode comprises at least one of a sodium metal negative electrode or a sodium-supplemented negative electrode; the sodium-supplemented negative electrode comprises at least one of sodium-supplemented hard carbon or sodium-supplemented soft carbon; the current collector of the sodium-supplemented negative electrode is aluminum foil;

[0025] And / or, the current collector of the positive electrode is aluminum foil.

[0026] In some embodiments, in step (1), the areal capacity of the positive electrode is lower than the areal capacity of the negative electrode; and the ratio of the areal capacity of the positive electrode to the areal capacity of the negative electrode is 1:1.1~1.2.

[0027] In some embodiments, in step (2), the constant voltage discharge includes one or more segments of constant voltage discharge;

[0028] And / or, the constant voltage discharge time is 0.3-4h;

[0029] And / or, the constant voltage discharge voltage is 0.8-2.0V.

[0030] Embodiments of the present invention provide a sodium-ion battery, including a sodium-ion battery positive electrode interface film according to embodiments of the present invention, or a sodium-ion battery positive electrode interface film prepared by a preparation method according to embodiments of the present invention. The advantages of the sodium-ion battery including the sodium-ion battery positive electrode interface film in the embodiments of the present invention are not further elaborated here. Beneficial effects

[0031] The advantages and technical effects of the sodium-ion battery positive electrode interface film of the embodiment of the present invention are as follows: By pre-discharging the assembled sodium-ion battery to the reduction potential of the electrolyte and then performing constant-voltage discharge, the electrolyte can be reduced on the positive electrode surface, forming a positive electrode-electrolyte interface film containing inorganic substances. Unlike the loose and porous CEI membrane rich in organic matter formed by the initial oxidation of the electrolyte, the interface film of the present invention has a denser structure, reduces the solubility of the interface film, stabilizes transition metal ions, alleviates the structural and interfacial damage of the positive electrode material during the cycle process, and effectively improves the self-discharge performance, cycle performance, and coulombic efficiency of the sodium-ion battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] FIG1 is a schematic diagram of a method for forming a multi-component CEI on the surface of a positive electrode of a sodium ion battery by using an electrolyte containing elemental F, B, and P additives according to the present invention.

[0033] Figure 2 is a diagram of NaFe in the present invention. 1 / 3 Ni 1 / 3 Mn 1 / 3 Pre-discharge curves of O2 / / Na (NT / / Na) batteries using conventional carbonate electrolyte.

[0034] FIG3 is a NaFe 1 / 3 Ni 1 / 3 Mn 1 / 3 Pre-discharge curves of O2 / / Na (NT / / Na) batteries using the first, second, and third electrolytes.

[0035] FIG4 is a graph showing changes in discharge specific capacity and coulombic efficiency of Example 1 of the present invention and Comparative Example 1.

[0036] FIG5 is a graph showing changes in discharge specific capacity and coulombic efficiency of Example 2 of the present invention and Comparative Example 2.

[0037] FIG6 is a graph showing changes in discharge specific capacity and coulombic efficiency of Example 3 of the present invention and Comparative Example 3.

[0038] FIG7 is a graph showing changes in discharge specific capacity and coulombic efficiency of Example 4 of the present invention and Comparative Example 4.

[0039] FIG8 is a specific capacity-voltage curve diagram of the constant voltage stage of Example 2-4 of the present invention.

[0040] Reference numerals: negative electrode 1; positive electrode 2; electrolyte 3; current collector 4; sodium ion battery positive electrode interface film 5. Best Mode for Carrying Out the Invention

[0041] The embodiments of the present invention are described in detail below, and examples of the embodiments are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and intended to be used to explain the present invention, but should not be understood as limiting the present invention.

[0042] As shown in FIG1 , a positive electrode interface film 5 of a sodium ion battery according to an embodiment of the present invention is formed by pre-discharging the assembled sodium ion battery to the reduction potential of the electrolyte 3 and performing constant voltage discharge, so that the electrolyte 3 is reduced on the surface of the positive electrode 2, thereby forming a positive electrode interface film 5 in situ.

[0043] The sodium-ion battery positive electrode interface film of the present invention is prepared by pre-discharging the assembled sodium-ion battery to the reduction potential of the electrolyte and then performing constant-voltage discharge. This reduces the electrolyte on the positive electrode surface, forming a positive electrode-electrolyte interface film containing inorganic substances. Unlike the loose, porous CEI membrane rich in organic matter formed by initial oxidation of the electrolyte, the interface film of the present invention has a denser structure, reduces the solubility of the interface film, stabilizes transition metal ions, and alleviates structural and interfacial damage to the positive electrode material during cycling, effectively improving the self-discharge performance, cycling performance, and coulombic efficiency of the sodium-ion battery.

[0044] In the embodiments of the present invention, constant-voltage discharge at a reduction potential promotes full electrolyte reduction, correspondingly generating abundant reduction products and forming a dense CEI film. If constant-voltage discharge is not performed at a reduction potential, if the constant voltage is too low, the positive electrode structure may be damaged, reducing the battery's specific capacity. If the constant voltage is too high, the electrolyte reduction reaction is insufficient, resulting in a low number of reduction products and an inability to form a dense CEI film to protect the positive electrode surface.

[0045] In some embodiments, the sodium ion battery positive electrode interface film 5 is a multi-component sodium ion battery positive electrode interface film.

[0046] In some embodiments, the sodium-ion battery positive electrode interface film 5 includes at least one of a fluorine-containing organic compound, sodium fluoride, a boron-containing inorganic compound, and a phosphorus-containing inorganic compound. Preferably, the sodium-ion battery positive electrode interface film 5 includes a fluorine-containing organic compound, sodium fluoride, a boron-containing inorganic compound, and a phosphorus-containing inorganic compound. In the embodiment of the present invention, the electrolyte is fully reduced on the positive electrode surface to pre-form a multi-component CEI having a fluorine-containing organic compound, sodium fluoride, a boron-containing inorganic compound, a phosphorus-containing inorganic compound, etc., thereby stabilizing the positive electrode interface during the cycle process and effectively improving the self-discharge performance, cycle life, and coulombic efficiency of the sodium-ion battery.

[0047] In some embodiments, the electrolyte 3 includes an additive, and the additive includes at least one of a fluorine-containing additive, a boron-containing additive, or a phosphorus-containing additive; the additive is reduced by constant-voltage discharge to generate at least one of a fluorine-containing organic substance, sodium fluoride, a boron-containing inorganic substance, and a phosphorus-containing inorganic substance. For example, the fluorine-containing additive is reduced by constant-voltage discharge to generate a fluorine-containing organic substance and / or sodium fluoride, the boron-containing additive is reduced by constant-voltage discharge to generate a boron-containing inorganic substance, and the phosphorus-containing additive is reduced by constant-voltage discharge to generate a phosphorus-containing inorganic substance.

[0048] In an embodiment of the present invention, by selecting a fluorine-containing additive, a boron-containing additive or a phosphorus-containing additive as an electrolyte additive, the electrolyte can be fully reduced on the positive electrode surface by a constant voltage discharge method, and a multi-component positive electrode-electrolyte interface film having fluorine-containing organic matter, sodium fluoride, boron-containing inorganic matter, phosphorus-containing inorganic matter, etc. is pre-formed. The structure is denser, the solubility of the interface film is reduced, and the transition metal ions can be stabilized, thereby alleviating the structural and interfacial damage of the positive electrode material during the cycle process, and effectively improving the self-discharge performance, cycle performance, and coulombic efficiency of the sodium ion battery.

[0049] In some embodiments, the fluorine-containing additive includes at least one of fluoroethylene carbonate (FEC), bisfluoroethylene carbonate (DFEC), trifluoropropylene carbonate (TFPC), and sodium bistrifluoromethanesulfonimide (NaTFSI), preferably FEC;

[0050] and / or, the boron-containing additive includes at least one of sodium tetrafluoroborate (NaBF4) and sodium difluorooxalatoborate (NaDFOB), preferably NaBF4;

[0051] And / or, the phosphorus-containing additive includes at least one of trimethyl phosphate (TMP), triethyl phosphate (TEP), and tris(2,2,2-trifluoroethyl) phosphate (TFEP), preferably TEP.

[0052] In the embodiment of the present invention, by selecting the types of fluorine-containing additives, boron-containing additives, and phosphorus-containing additives, preferential reduction of the additives can be achieved while taking into account the cost of the additives, which is conducive to the pre-formation of a multi-component positive electrode-electrolyte interface film having fluorine-containing organic matter, sodium fluoride, boron-containing inorganic matter, phosphorus-containing inorganic matter, etc.

[0053] In some embodiments, the additive includes fluoroethylene carbonate; the volume percentage of the fluoroethylene carbonate in the electrolyte is 2-10%, specifically, for example, 2%, 3%, 5%, 8%, 10%;

[0054] And / or, the additives include fluoroethylene carbonate and sodium tetrafluoroborate; the volume percentage of the fluoroethylene carbonate in the electrolyte is 2-10%, specifically, for example, 2%, 3%, 5%, 8%, 10%, and the mass percentage of the sodium tetrafluoroborate in the electrolyte is 0.5-5%, specifically, for example, 0.5%, 1%, 2%, 3%, 4%, 5%;

[0055] And / or, the additives include fluoroethylene carbonate, sodium tetrafluoroborate and triethyl phosphate; the volume percentage of the fluoroethylene carbonate in the electrolyte is 2-10%, specifically, for example, 2%, 3%, 5%, 8%, 10%, the mass percentage of the sodium tetrafluoroborate in the electrolyte is 0.5-5%, specifically, for example, 0.5%, 1%, 2%, 3%, 4%, 5%, and the volume percentage of the triethyl phosphate in the electrolyte is 5-15%, specifically, for example, 5%, 8%, 10%, 12%, 15%.

[0056] In the embodiments of the present invention, further optimizing the composition of the additives is beneficial to further improve the performance of the positive electrode interface film, and further improve the self-discharge performance, cycle life, and coulombic efficiency of the sodium ion battery.

[0057] In some embodiments, the positive electrode 2 includes a layered transition metal oxide; the layered transition metal oxide includes NaFe 1 / 3 Ni 1 / 3 Mn 1 / 3 O2、Na 2 / 3 Ni 1 / 3 Mn 2 / 3 O2、NaFe 0.5 Mn 0.5 At least one of O2, preferably NaFe 1 / 3 Ni 1 / 3 Mn 1 / 3 O2;

[0058] And / or, the electrolyte 3 includes a sodium salt and a solvent; the sodium salt includes at least one of sodium hexafluorophosphate (NaPF6) or sodium perchlorate (NaClO4); the concentration of the sodium salt in the electrolyte 3 is 0.8-1.5 M, specifically, for example, 0.8 M, 1.0 M, 1.2 M, 1.5 M;

[0059] The solvent comprises a carbonate ester; the carbonate ester includes at least one of ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), propylene carbonate (PC), and diethyl carbonate (DEC). Preferably, the volume ratio of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate is 1:1:1. In embodiments of the present invention, the carbonate solvent exhibits stable electrochemical performance, meeting the stable electrochemical window required for sodium-ion battery charge and discharge. Layered transition metal oxide materials are simple to synthesize, have substantial capacity, and exhibit a high voltage platform.

[0060] In some embodiments, the preparation method of the electrolyte 3 includes uniformly mixing a sodium salt, a solvent, and an additive to obtain the electrolyte 3.

[0061] As shown in FIG1 , a method for preparing a positive electrode interface film 5 of a sodium ion battery according to an embodiment of the present invention comprises the following steps:

[0062] (1) Assemble a battery by combining the positive electrode 2, the negative electrode 1 and the electrolyte 3;

[0063] (2) The battery is pre-discharged to the reduction potential of the electrolyte 3, and then discharged at a constant voltage to obtain a sodium ion battery positive electrode interface film 5.

[0064] In an embodiment of the present invention, by pre-discharging the assembled sodium-ion battery to the reduction potential of the electrolyte and then performing constant-voltage discharge, the electrolyte can be reduced on the positive electrode surface, forming a positive electrode-electrolyte interface film containing inorganic substances. Unlike the loose, porous CEI film rich in organic matter formed by the initial oxidation of the electrolyte, the interface film structure of the present invention is more compact, reducing the solubility of the interface film, stabilizing transition metal ions, and alleviating structural and interfacial damage of the positive electrode material during the cycle process, effectively improving the self-discharge performance, cycle performance, and coulombic efficiency of the sodium-ion battery.

[0065] In some embodiments, in step (1), the negative electrode 1 comprises at least one of a sodium metal negative electrode or a sodium-supplemented negative electrode; the sodium-supplemented negative electrode comprises at least one of sodium-supplemented hard carbon (HC) or sodium-supplemented soft carbon; the sodium-supplemented negative electrode is provided with a sodium-supplemented method comprising at least one of a physical method, a chemical method, and an electrochemical method, preferably an electrochemical method; and the current collector 4 of the sodium-supplemented negative electrode is aluminum foil. In embodiments of the present invention, the sodium-supplemented carbon negative electrode material has a low sodium storage platform and a high sodium storage capacity. The sodium-supplemented carbon negative electrode is coated on the aluminum foil current collector to ensure the stability of the current collector at low voltage. The sodium-supplemented method is preferably an electrochemical method to ensure uniform sodium supplementation.

[0066] In some embodiments, in step (1), the preparation of the sodium-supplemented hard carbon negative electrode includes: using a hard carbon electrode as a negative electrode, sodium metal as a counter electrode, adding electrolyte 3 and a separator, assembling a 2025-type button battery, and performing constant current discharge using a battery tester at room temperature to obtain a sodium-supplemented hard carbon negative electrode by an electrochemical method.

[0067] In some embodiments, in step (1), the current collector 4 of the positive electrode 2 is aluminum foil. In the embodiment of the present invention, the positive electrode material is coated on the aluminum foil current collector to ensure the stability of the current collector under high voltage.

[0068] In some embodiments, in step (1), the areal capacity of the positive electrode 2 is lower than the areal capacity of the negative electrode 1; the ratio of the areal capacity of the positive electrode 2 to the areal capacity of the negative electrode 1 is 1:1.1 to 1.2, specifically, for example, 1:1.1, 1:1.15, 1:1.2. In the embodiments of the present invention, by optimizing the areal capacity of the positive and negative electrodes, the precipitation of sodium metal on the negative electrode side due to excessive positive electrode is prevented.

[0069] In some embodiments, in step (2), the constant voltage discharge includes one or more constant voltage discharges, specifically, for example, one, two, or three stages, with the reduction potential of each stage being the same or different. In an embodiment of the present invention, the battery is pre-discharged to the reduction potential of the electrolyte, and a single or multiple constant voltage discharge is performed for a certain period of time to promote the full reduction of the electrolyte on the positive electrode surface to form a stable multi-component CEI.

[0070] In some embodiments, in step (2), the constant voltage discharge time is 0.3-4 hours, specifically, for example, 0.3 hours, 0.5 hours, 1 hour, 2 hours, 3 hours, 4 hours;

[0071] And / or, the constant voltage discharge voltage is 0.8-2.0V, specifically, for example, 0.8V, 1V, 1.05V, 1.1V, 1.23V, 1.3V, 1.5V, 1.8V, 2.0V, preferably, 1.0-1.3V.

[0072] In the embodiments of the present invention, optimizing the constant-voltage discharge time and voltage can further improve the self-discharge performance, cycle performance, and coulombic efficiency of the sodium-ion battery. When the constant-voltage discharge time is too short or the constant-voltage voltage is too high, the reduction reaction of the electrolyte is insufficient, the reduction product is small, and a dense CEI film cannot be formed to protect the positive electrode surface. When the constant-voltage discharge time is too long or the constant-voltage voltage is too low, the positive electrode structure itself may be damaged to a certain extent, reducing the specific capacity of the battery.

[0073] In some embodiments, in step (2), the battery is pre-discharged with a small current to the reduction potential of the electrolyte 3; preferably, the current density of the small current discharge is 5-10 mA / g, specifically, for example, 5 mA / g, 7 mA / g, 9 mA / g, 10 mA / g. In the embodiment of the present invention, pre-discharging with a small current to the reduction potential of the electrolyte is beneficial to prevent excessive polarization caused by large current pre-discharge, which may cause a reduction potential shift.

[0074] A sodium ion battery according to an embodiment of the present invention includes a sodium ion battery positive electrode interface film 5 according to an embodiment of the present invention or a sodium ion battery positive electrode interface film 5 prepared by a preparation method according to an embodiment of the present invention. In the embodiment of the present invention, all advantages of the sodium ion battery including the sodium ion battery positive electrode interface film are not further described here.

[0075] In some embodiments, after the sodium ion battery positive electrode interface film 5 is prepared, the battery is charged and the battery cycle charge and discharge voltage range is controlled to put the sodium ion battery into a normal charge and discharge state. Modes for Carrying Out the Invention

[0076] The present invention is described below with reference to specific examples. It should be noted that these examples are merely illustrative and do not limit the present invention in any way.

[0077] Configuration of the first electrolyte, the second electrolyte, and the third electrolyte and determination of reduction potential:

[0078] 1. The configuration of electrolyte 3 includes:

[0079] (1) Preparation of conventional carbonate electrolyte: In a glove box filled with high-purity argon, prepare conventional carbonate electrolyte 1.0 M NaPF6 / EC:DMC:EMC with a solvent volume ratio of 1:1:1, stir evenly and let it stand for 12 hours.

[0080] (2) Preparation of the first electrolyte: In a glove box filled with high-purity argon, add 5% by volume of FEC to the conventional carbonate electrolyte, stir evenly and let it stand for 12 hours to obtain the first electrolyte.

[0081] (3) Preparation of the second electrolyte: In a glove box filled with high-purity argon, add 1% by mass of NaBF4 to the first electrolyte, stir evenly and let it stand for 12 hours to obtain the second electrolyte.

[0082] (4) Preparation of the third electrolyte: In a glove box filled with high-purity argon, 10% by volume of TEP was added to the second electrolyte, stirred evenly, and allowed to stand for 12 hours to obtain the third electrolyte. The sodium salt, organic solvent, and additives used in the above preparation were all battery grade.

[0083] 2. NaFe 1 / 3 Ni 1 / 3 Mn 1 / 3 Preparation of O2 positive electrode: NaFe 1 / 3 Ni 1 / 3 Mn 1 / 3 The O2 positive electrode was mixed with polyvinylidene fluoride and conductive agent in a mass ratio of 94%:3%:3%, and evenly dispersed in N-methyl-2-pyrrolidone solvent and ground to form a uniform slurry. The slurry was evenly coated on aluminum foil using a film applicator and dried in a vacuum oven at 100 ° C for 12 hours. The NaFe 1 / 3 Ni 1 / 3 Mn 1 / 3 O2 positive electrode.

[0084] 3. Measurement of reduction potential: In a glove box filled with high-purity argon, 1 / 3 Ni 1 / 3 Mn 1 / 3An O2 electrode served as the positive electrode (2), sodium metal as the counter electrode, and electrolyte (3), a separator, and other components were added to form a 2025-type button cell. Constant-current discharge was performed at room temperature using a battery tester to evaluate the reduction potentials of a conventional carbonate electrolyte and three other electrolytes on the positive electrode (2) surface.

[0085] As shown in Figures 2 and 3, the main reduction potential of the conventional carbonate electrolyte is 0.5-1.0 V, the main reduction potential of the first electrolyte, the second electrolyte, and the third electrolyte is above 1.0 V, and the reduction potential range is 1.0-1.3 V, while the NaFe 1 / 3 Ni 1 / 3 Mn 1 / 3 There is no obvious decomposition reaction of the O2 positive electrode above 0.5 V, indicating that constant-voltage discharge at the reduction potential of electrolyte 3 will not cause structural damage to the positive electrode 2 itself.

[0086] Example 1:

[0087] The first step is to use the above NaFe 1 / 3 Ni 1 / 3 Mn 1 / 3 An O2 electrode was used as positive electrode 2, sodium metal was used as negative electrode 1, the first electrolyte was electrolyte 3, and Celgard® 2400 was used as the separator. A 2025-type NT / / Na button cell was assembled in a glove box. The areal capacity of the positive electrode 2 was much lower than that of the negative electrode 1, resulting in an excess capacity of the negative electrode 1.

[0088] In the second step, at room temperature, the prepared battery was pre-discharged at a constant current of 5 mA / g using a battery tester until the reduction potential reached 1.1 V, and then immediately discharged at a constant voltage of 1.1 V for 1 hour to obtain the positive electrode interface film of the sodium ion battery.

[0089] The third step is to immediately charge the battery with constant current. First, charge and discharge it at a current density of 0.1C (1C=120 mA / g) for 3 cycles, then charge and discharge it at 0.5C for 85 cycles, let it stand for 100 hours, and then continue to charge and discharge it at a current density of 0.5C for 12 cycles. The test voltage range is 2.0~4.0 V.

[0090] Comparative Example 1:

[0091] The method is the same as Example 1, except that the second step is omitted, that is, the pre-discharge and constant voltage discharge steps are not performed.

[0092] FIG4 shows the changes in discharge specific capacity and efficiency of the sodium ion batteries in Example 1 and Comparative Example 1.

[0093] Example 2:

[0094] The first step is to use the above NaFe 1 / 3 Ni 1 / 3 Mn 1 / 3An O2 electrode was used as positive electrode 2, a sodium-supplemented hard carbon electrode was used as negative electrode 1, the first electrolyte was electrolyte 3, and Celgard® 2400 was used as a separator. A 2025-type NT / / HC button cell was assembled in a glove box. The ratio of the areal capacity of the positive electrode 2 to the areal capacity of the negative electrode 1 was 1:1.15.

[0095] The preparation of sodium-supplemented hard carbon electrode includes:

[0096] (1) Preparation of hard carbon anode: Mix hard carbon and sodium alginate binder in a mass ratio of 95.5%:4.5%, add deionized water and grind to form a uniform slurry. Use a film applicator to evenly coat it on aluminum foil by blade coating, dry it in a vacuum oven at 100℃ for 12 hours, and slice it to obtain a hard carbon anode electrode.

[0097] (2) Preparation of sodium-supplemented hard carbon anode: In a glove box filled with high-purity argon, a hard carbon electrode was used as the anode 1, sodium metal as the counter electrode, and electrolyte 3, a separator, and other components were added to assemble a 2025-type button cell. At room temperature, a constant current discharge was performed using a battery tester for a certain period of time. The battery was then disassembled to obtain the electrochemical sodium-supplemented anode.

[0098] In the second step, at room temperature, the prepared battery was pre-discharged at a constant current of 5 mA / g using a battery tester until the reduction potential reached 1.1 V, and then immediately discharged at a constant voltage of 1.1 V for 1 hour to obtain the positive electrode interface film of the sodium ion battery.

[0099] The third step is to immediately charge the battery with constant current, first charging and discharging at a current density of 0.1C (1C=120 mA / g) for 3 cycles, and then charging and discharging at 0.5C for 397 cycles, with a test voltage range of 1.0~4.0 V.

[0100] Comparative Example 2:

[0101] The method is the same as Example 2, except that the second step is omitted, that is, the pre-discharge and constant voltage discharge steps are not performed.

[0102] FIG5 shows the changes in discharge specific capacity and efficiency of the sodium ion batteries in Example 2 and Comparative Example 2.

[0103] Example 3:

[0104] The first step is to use the above NaFe 1 / 3 Ni 1 / 3 Mn 1 / 3 An O2 electrode served as positive electrode 2, a sodium-supplemented hard carbon electrode served as negative electrode 1, the second electrolyte served as electrolyte 3, and Celgard® 2400 served as the separator. A 2025-type NT / / HC button cell was assembled in a glove box. The ratio of the areal capacity of the positive electrode 2 to the areal capacity of the negative electrode 1 was 1:1.15. The sodium-supplemented hard carbon electrode was the same as that used in Example 2.

[0105] In the second step, at room temperature, the prepared battery was pre-discharged at a constant current of 5 mA / g using a battery tester until the reduction potential reached 1.05 V, and then immediately discharged at a constant voltage of 1.05 V for 1 hour to obtain the positive electrode interface film of the sodium ion battery.

[0106] The third step is to immediately charge the battery with constant current, first charging and discharging at a current density of 0.1C (1C=120 mA / g) for 3 cycles, and then charging and discharging at 0.5C for 397 cycles, with a test voltage range of 1.0~4.0 V.

[0107] Comparative Example 3:

[0108] The method is the same as Example 3, except that the second step is omitted, that is, the pre-discharge and constant voltage discharge steps are not performed.

[0109] FIG6 shows the changes in discharge specific capacity and efficiency of the sodium ion batteries in Example 3 and Comparative Example 3.

[0110] Example 4:

[0111] The first step is to use the above NaFe 1 / 3 Ni 1 / 3 Mn 1 / 3 An O2 electrode served as positive electrode 2, a sodium-supplemented hard carbon electrode served as negative electrode 1, a third electrolyte served as electrolyte 3, and Celgard® 2400 served as a separator. A 2025 NT / / HC button cell was assembled in a glove box. The ratio of the areal capacity of the positive electrode 2 to the areal capacity of the negative electrode 1 was 1:1.15. The sodium-supplemented hard carbon electrode was the same as that used in Example 2.

[0112] In the second step, at room temperature, the prepared battery was pre-discharged at a constant current of 5 mA / g using a battery tester. After the reduction potential reached 1.23 V, the battery was immediately discharged at a constant voltage of 1.23 V for 0.5 hours. The battery was further discharged to a reduction potential of 1.05 V, and then the battery was immediately discharged at a constant voltage of 1.05 V for 0.5 hours to obtain the positive electrode interface film of the sodium ion battery.

[0113] The third step is to immediately charge the battery with constant current, first charging and discharging at a current density of 0.1C (1C=120 mA / g) for 3 cycles, and then charging and discharging at 0.5C for 397 cycles, with a test voltage range of 1.0~4.0 V. Industrial Applicability

[0114] Comparative Example 4:

[0115] The same as Example 4, except that the second step is omitted, that is, the pre-discharge and constant voltage discharge steps are not performed.

[0116] FIG7 shows the changes in discharge specific capacity and efficiency of the sodium ion batteries in Example 4 and Comparative Example 4.

[0117] FIG8 is a specific capacity-voltage curve obtained during the constant voltage stage in Example 2-4.

[0118] The performance test results of the sodium ion batteries of Examples 1-4 and Comparative Examples 1-4 are shown in Table 1.

[0119] Table 1 Performance of sodium ion batteries in Examples 1-4 and Comparative Examples 1-4

[0120]

[0121] Compared with Comparative Example 1, the use of F-containing additive FEC in Example 1 can significantly improve the self-discharge performance of the NT / / Na battery through single-stage constant-voltage discharge, and has certain improvements on the capacity retention rate and average coulombic efficiency. This shows that pre-discharge to the reduction potential of the first electrolyte and constant-voltage discharge can help FEC to be fully reduced to form F-containing organic matter, NaF, etc., and stabilize the positive electrode surface.

[0122] Compared with Comparative Example 2, the average coulombic efficiency of the NT / / HC battery using the F additive FEC in Example 2 is improved, but the capacity retention rate is not significantly improved, indicating that the CEI with fewer components formed by the F additive FEC has limited effect in the more demanding sodium ion full battery.

[0123] Compared with Example 2 and Comparative Example 3, Example 3 simultaneously used F-containing additive FEC and B-containing additive NaBF4. Through single-stage constant voltage discharge, the capacity retention rate and average coulombic efficiency of the NT / / HC battery were significantly improved, indicating that the CEI of multiple components such as F-containing organic matter, NaF, and B-containing inorganic matter generated by reduction of the second electrolyte in Example 3 has a significant effect on improving the cycle stability and average coulombic efficiency of the sodium ion full battery.

[0124] Compared to Examples 2-3 and Comparative Example 4, the third electrolyte of Example 4 simultaneously uses F-containing additive FEC, B-containing additive NaBF4, and P-containing additive TEP. Through two-stage constant voltage discharge, the capacity retention rate of the NT / / HC battery can be further improved. After 400 cycles, the capacity retention rate reaches 71.0%, and the average coulombic efficiency is 99.3%. Among all comparative examples and examples, the sodium ion battery in Example 4 has the best overall performance. This shows that the multi-component CEI formed by the reduction of F-containing organic matter, NaF, B-containing inorganic matter, and P-containing inorganic matter has a more compact structure. Compared with the CEI with fewer components in Examples 1-3, it can further reduce the solubility of the interfacial film, stabilize the transition metal ions, and alleviate the structural and interfacial damage of the positive electrode material during the cycle, greatly improving the performance of the sodium ion battery.

[0125] In the present invention, the terms "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" mean that the specific features, structures, materials, or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic expressions 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, those skilled in the art can combine and combine different embodiments or examples described in this specification and the features of different embodiments or examples without contradiction.

[0126] Although the above embodiments have been shown and described, it is understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. Changes, modifications, substitutions and variations of the above embodiments by those skilled in the art are all within the scope of protection of the present invention.

Claims

1. A sodium ion battery positive electrode interface film, characterized in that: The assembled sodium ion battery is pre-discharged to the reduction potential of the electrolyte and then discharged at a constant voltage, so that the electrolyte is reduced on the surface of the positive electrode and a positive electrode interface film is formed in situ; The electrolyte includes additives, and the additives include at least one of fluorine-containing additives, boron-containing additives or phosphorus-containing additives; the fluorine-containing additives include at least one of fluoroethylene carbonate, difluoroethylene carbonate, trifluoropropylene carbonate, and sodium bistrifluoromethanesulfonimide; the boron-containing additives include at least one of sodium tetrafluoroborate and sodium difluorooxalatoborate; the phosphorus-containing additives include at least one of trimethyl phosphate, triethyl phosphate, and tris (2,2,2-trifluoroethyl) phosphate.

2. The sodium ion battery positive electrode interface film according to claim 1, characterized in that The sodium ion battery positive electrode interface film includes at least one of fluorine-containing organic matter, sodium fluoride, boron-containing inorganic matter and phosphorus-containing inorganic matter.

3. The sodium ion battery positive electrode interface film according to claim 1, characterized in that The positive electrode comprises a layered transition metal oxide; the layered transition metal oxide comprises NaFe 1 / 3 Ni 1 / 3 Mn 1 / 3 O2、Na 2 / 3 Ni 1 / 3 Mn 2 / 3 O2、NaFe 0.5 Mn 0.5 At least one of O2; And / or, the electrolyte comprises a sodium salt and a solvent; the sodium salt comprises at least one of sodium hexafluorophosphate or sodium perchlorate; the concentration of the sodium salt in the electrolyte is 0.8-1.5 M; the solvent comprises a carbonate; the carbonate comprises at least one of ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, propylene carbonate, and diethyl carbonate.

4. The sodium ion battery positive electrode interface film according to claim 1, characterized in that The additive includes fluoroethylene carbonate; the volume percentage of the fluoroethylene carbonate in the electrolyte is 2-10%; And / or, the additive comprises fluoroethylene carbonate and sodium tetrafluoroborate; the volume percentage of the fluoroethylene carbonate in the electrolyte is 2-10%, and the mass percentage of the sodium tetrafluoroborate in the electrolyte is 0.5-5%; And / or, the additives include fluoroethylene carbonate, sodium tetrafluoroborate and triethyl phosphate; the volume percentage of the fluoroethylene carbonate in the electrolyte is 2-10%, the mass percentage of the sodium tetrafluoroborate in the electrolyte is 0.5-5%, and the volume percentage of the triethyl phosphate in the electrolyte is 5-15%.

5. A method for preparing a positive electrode interface film for a sodium ion battery according to any one of claims 1 to 4, characterized in that: The following steps are involved: (1) Assemble the battery by combining the positive electrode, negative electrode and electrolyte; (2) The battery is pre-discharged to the reduction potential of the electrolyte, and then discharged at a constant voltage to obtain a positive electrode interface film of the sodium ion battery.

6. The method for preparing a positive electrode interface film for a sodium ion battery according to claim 5, characterized in that: In the step (1), the negative electrode comprises at least one of a sodium metal negative electrode or a sodium-supplemented negative electrode; the sodium-supplemented negative electrode comprises at least one of sodium-supplemented hard carbon or sodium-supplemented soft carbon; the current collector of the sodium-supplemented negative electrode is aluminum foil; And / or, the current collector of the positive electrode is aluminum foil.

7. The method for preparing a positive electrode interface film for a sodium ion battery according to claim 5, characterized in that: In the step (1), the surface capacity of the positive electrode is lower than the surface capacity of the negative electrode; and the ratio of the surface capacity of the positive electrode to the surface capacity of the negative electrode is 1:1.1-1.

2.

8. The method for preparing a positive electrode interface film for a sodium ion battery according to claim 5, characterized in that: In the step (2), the constant voltage discharge includes one or more constant voltage discharges; And / or, the constant voltage discharge time is 0.3-4h; And / or, the voltage of the constant voltage discharge is 0.8-2.0V.

9. A sodium ion battery, characterized in that: The invention comprises the sodium ion battery positive electrode interface film according to any one of claims 1 to 4 or the sodium ion battery positive electrode interface film prepared by the preparation method according to any one of claims 5 to 8.

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