Sodium ion battery negative electrode material, pretreatment method and prepared sodium ion battery

Abstract

The present invention relates to a sodium-ion battery anode material, a pretreatment method for a sodium-ion battery anode material, and a corresponding sodium-ion battery based thereon. The provided anode material is pretreated to form a layer of extremely low-solubility SEI on the surface, which is then used as the anode material for a commercial sodium-ion battery. The treated anode material has a stable and insoluble SEI. The resulting sodium-ion battery achieves an initial coulombic efficiency of 90.29% and an average coulombic efficiency of 99.95% after 900 cycles.

Description

Technical Field

[0001] The present invention relates to a sodium ion battery negative electrode material, a pretreatment method and a prepared sodium ion battery, belonging to the field of sodium ion batteries. Background Art

[0002] Energy is a key driver of social development. Since fossil fuels are non-renewable, developing alternative energy systems is crucial. These include solar, wind, and chemical energy. However, solar and wind energy struggle to provide stable and continuous energy, and their equipment is expensive. Therefore, chemical energy has become the most practical alternative. Among chemical energy sources, lithium-ion batteries (LIBs) have garnered widespread attention due to their high energy density and long cycle life, and are currently being successfully applied in portable electronic devices. However, the recent explosive growth in demand for LIBs in electric vehicles and large-scale energy storage grids has created an increasingly significant supply-demand imbalance with the limited, unevenly distributed, and high-priced lithium resources in the Earth's crust. Therefore, it is necessary to find alternatives to LIBs. Sodium reserves are abundant, and LIBs share a similar operating mechanism to LIBs, allowing for commercial production using existing LIB production lines. Therefore, LIBs represent the most promising secondary ion battery.

[0003] However, compared to lithium-ion batteries, sodium-ion batteries currently have a lower energy density, and the instability of the solid-electrolyte interphase (SEI) limits their long-term cycling performance, hindering further commercial application. The improvement of the energy density of sodium-ion batteries is mainly focused on the development of high-energy-density positive and negative electrode materials, such as alloyed negative electrodes. The instability of the electrode-electrolyte interface (SEI) is an important cause of battery performance degradation, and this problem is more prominent in sodium-ion batteries than in lithium-ion batteries. There is an urgent need for basic scientific mechanism analysis and breakthroughs in actual cycling performance. SEI is a passivation layer formed after the negative electrode contacts the electrolyte and the electrolyte is reduced. An ideal electrode-electrolyte interface should meet the following requirements: (1) allow Na ions to migrate to form a battery circuit; (2) be insulated from electrons to avoid side reactions and continuous consumption of sodium sources; and (3) electrolyte molecules cannot contact the electrode through the SEI, preventing further reactions between the electrode and the electrolyte. An unstable SEI can trigger a series of side reactions, leading to electrolyte depletion and sodium source consumption, reducing the battery's Coulombic efficiency, thereby affecting the battery's long-term cycling performance and lowering its actual capacity retention rate. Finally, an unstable SEI can have high and uneven interfacial impedance, hindering sodium ion migration and affecting the battery's rate performance. Therefore, interfacial instability is a key issue hindering battery performance improvements.

[0004] One of the main reasons why the SEI in sodium-ion batteries (SIBs) is less stable than in lithium-ion batteries is that it is more soluble in the electrolyte, making it incapable of meeting the long-term cycling requirements of SIBs. The dissolution of the SEI consumes the limited sodium source in the battery and accelerates electrolyte depletion, leading to irreversible capacity decay and even battery failure. Stabilizing the SEI on the surface of the anode material is a key challenge facing SIBs. Summary of the Invention

[0005] To address the issue of reduced performance of sodium-ion batteries due to SEI dissolution on the surface of sodium-ion battery anode materials, the present invention provides a sodium-ion battery anode material, a pretreatment method for sodium-ion battery anode materials, and a corresponding sodium-ion battery based on these. The provided anode material is pretreated to form a layer of SEI on the surface, which is then used as the anode material for commercial sodium-ion batteries. The treated anode material has a stable and insoluble SEI. The resulting sodium-ion battery achieves a first-cycle coulombic efficiency of 90.29%, and an average coulombic efficiency of 99.95% after 900 cycles.

[0006] The technical solution adopted is: a pretreatment method for sodium ion battery negative electrode materials, comprising the following steps:

[0007] Obtaining the negative electrode material body;

[0008] Constructing a pretreatment battery system: constructing a pretreatment battery system with the negative electrode material as the working electrode, the concentration of the electrolyte in the pretreatment battery system is greater than the concentration of the electrolyte in the target battery system, the pretreatment battery system is a battery system in which the negative electrode material is pretreated, the pretreatment battery system is a sodium ion battery, and the target battery system is a battery system to be constructed with the pretreated negative electrode material as the negative electrode;

[0009] Performing a charge-discharge cycle on the pre-processed battery system: performing a charge-discharge cycle on the constructed pre-processed battery system to form an electrode-electrolyte interface on the negative electrode material body;

[0010] The negative electrode material body is taken out to obtain the pretreated negative electrode material.

[0011] As a preferred embodiment, the pretreatment battery system is constructed in one of the following ways: (1) a half-battery system is constructed with the negative electrode material as the working electrode and sodium metal as the counter electrode; (2) a full-battery system is constructed with the negative electrode material as the negative electrode and the sodium battery positive electrode material as the positive electrode.

[0012] As a preferred solution, the concentration of the electrolyte in the pretreatment battery system is higher than 1.8 mol / L, preferably higher than 4 mol / L. At this high concentration, the electrode electrolyte interface formed has a very low organic content and a very high inorganic content. The SEI has extremely low solubility in the solvent and high interface stability.

[0013] As a preferred embodiment, the negative electrode material is a hard carbon electrode material or a copper electrode material. When the hard carbon electrode material is used as the negative electrode material, the sodium ion battery system is subjected to charge and discharge cycles at a current of (0.1-1) C (1 C = 300 mAh / g) and a voltage of (0.0002-2) V. A stable electrode-electrolyte interface can be formed during the charge and discharge cycles. When the copper electrode material is used as the negative electrode material, the sodium ion battery system is subjected to charge and discharge cycles at a current of (5-100) μA and a voltage of (0.0002-2) V. A stable electrode-electrolyte interface can be formed during the charge and discharge cycles. Further preferably, the pretreatment battery system is subjected to charge and discharge cycles for at least one cycle, preferably three or more cycles, and more preferably three cycles, to ensure a sufficient number of cycles to improve the first-cycle coulombic efficiency while also taking into account the ease of operation.

[0014] As a preferred embodiment, the electrolyte components in the pretreatment battery system are the same as or different from the electrolyte components in the target battery system, and the electrolyte of the pretreatment battery system and / or the target battery system includes a sodium salt and a solvent, the solvent is an organic solvent, and the sodium salt and the solvent meet at least one of the following conditions:

[0015] The sodium salt is a combination of one or more of sodium hexafluorophosphate, sodium bis(fluorosulfonyl)imide, sodium nitrate, sodium perchlorate, sodium bis(trifluoromethylsulfonyl)imide, sodium trifluoromethanesulfonate, sodium chloride, sodium tetrafluoroborate, sodium methanesulfonate, sodium trifluoromethylsulfonate, sodium bis(oxalatoborate), sodium difluorooxalatoborate, sodium difluorophosphate, sodium difluorobis(oxalatophosphate), and sodium hexafluoroarsenate, with sodium bis(fluorosulfonyl)imide, sodium hexafluorophosphate, and sodium perchlorate being more preferred. This type of sodium salt has high solubility in solvents and can form a highly concentrated electrolyte, which is conducive to the formation of a stable SEI.

[0016] ——The solvent is a combination of one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, propylene carbonate, diethyl carbonate, dioxolane, ethyl methyl carbonate, dimethyl carbonate, vinylene carbonate, fluoroethylene carbonate, and tetrahydrofuran; preferably, the sodium salt in the pretreatment battery system is sodium bis(fluorosulfonyl)imide, and the solvent is ethylene glycol dimethyl ether.

[0017] The preparation method of the hard carbon electrode is as follows: hard carbon powder, a binder (polyvinylidene fluoride (PVDF)), and a conductive agent (conductive carbon black) are uniformly mixed in a dispersant (N-methylpyrrolidone) according to a set ratio, and the mixture is coated on a conductive current collector; the coated hard carbon electrode is vacuum dried to obtain a hard carbon electrode.

[0018] As a preferred solution, the content of organic components in the components of the electrode-electrolyte interface is low, and the content of inorganic components is high. Specifically, in the interface components, the atomic weight content of carbon atoms is less than 60%, and preferably the atomic weight content of carbon atoms is less than 50%. Under this setting, the solubility of the electrode-electrolyte interface in the solvent and the electrolyte is extremely low. Even if immersed for more than 50 hours, the capacity loss of the interface can still be guaranteed to be less than 1.5μAh.

[0019] In another aspect, the present invention provides a sodium-ion battery negative electrode material, wherein the negative electrode material body comprises a preformed electrode-electrolyte interface layer on its surface. The electrode-electrolyte interface is formed in a high-concentration electrolyte, preferably having an organic component content of less than 70%. After the electrode-electrolyte interface is immersed in an organic solvent for 50 hours, the component loss rate within the electrode-electrolyte interface is less than 15%. The carbon content of the electrode-electrolyte interface components is less than 60%, preferably less than 50%. The lower organic component and higher inorganic component effectively inhibit the dissolution of the interface components in the electrolyte, thereby ensuring the stability of the interface. When the carbon content is less than 50%, the carbon content of the interface is almost unchanged during immersion in the electrolyte. When the carbon content is 64%, the carbon content is reduced by approximately 24% under the same immersion conditions. Therefore, ensuring that the carbon content is less than 50% can significantly improve the stability of the interface.

[0020] The sodium ion battery negative electrode mentioned in the above scheme can be obtained by the pretreatment method of the above negative electrode material.

[0021] On the other hand, the present invention also provides a sodium ion battery, which is constructed by using the pretreated sodium ion battery negative electrode material prepared in the above scheme as the negative electrode, specifically a commercial sodium ion full battery, using a commercial electrolyte with a concentration of 1 mol / L, and more preferably, using a hard carbon electrode as the negative electrode and a manganese iron nickel based layered oxide as the positive electrode to construct the full battery. The preparation process is shown in Figure 1 First, a hard carbon electrode was assembled with sodium metal and a 4 mol / L ethylene glycol dimethyl ether solution of sodium bis(fluorosulfonyl)imide to form a battery for pretreatment of the negative electrode material. After the battery was cycled three times, the negative electrode material was removed and assembled into a full battery according to the above-mentioned commercial sodium ion full battery assembly method.

[0022] The beneficial effects of the present invention include: After pretreatment, the negative electrode material forms a stable, insoluble electrode-electrolyte interface layer with a high concentration of inorganic components and a low concentration of organic components, making the entire electrode-electrolyte interface insoluble in the electrolyte. The resulting full battery achieves a capacity retention rate of 80% and an average coulombic efficiency of 99.95% after 900 charge-discharge cycles. More detailed beneficial effects are described in the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of the assembly process of a full battery;

[0024] Figure 2 Capacity loss of batteries corresponding to different negative electrode materials;

[0025] Figure 3 Cumulative capacity loss of batteries corresponding to different negative electrode materials;

[0026] Figure 4 Normalized sodium ion concentration of different anode materials after immersion in solvent;

[0027] Figure 5 Element content of different negative electrode materials;

[0028] Figure 6 Changes in element content of different negative electrode materials after immersion in electrolyte;

[0029] Figure 7 (a), Figure 7 (b) and Figure 7 (c) show the dissolution of the electrode-electrolyte interface at different concentrations;

[0030] Figure 8 The first week charge and discharge curves of the full battery with and without pretreatment of the negative electrode material;

[0031] Figure 9 Long cycle of full battery corresponding to negative electrode materials with and without pretreatment;

[0032] Figure 10 Coulombic efficiency of the full battery with and without pretreatment of the negative electrode material. DETAILED DESCRIPTION

[0033] The present invention will be further explained in detail below with reference to the accompanying drawings and specific embodiments. However, it should be understood that the protection scope of the present invention is not limited to the specific embodiments.

[0034] In order to improve the stability of sodium ion batteries, the present invention pre-treats the negative electrode materials of sodium ion batteries and assembles sodium ion batteries with the pre-treated negative electrode materials. Compared with untreated sodium ion batteries, the coulombic efficiency is greatly improved.

[0035] The pretreatment method of the negative electrode material of the present invention comprises the following steps: (1) obtaining the negative electrode material body; (2) constructing a pretreatment battery system; (3) subjecting the pretreatment battery system to charge and discharge cycles; and (4) removing the negative electrode material body to obtain a pretreated negative electrode material. Assembling a target battery with the pretreated negative electrode material can improve the performance of the target battery. The pretreatment method is further described below.

[0036] Obtaining the anode material: The anode material used in conventional sodium-ion batteries can be purchased directly or prepared in-house. For superior performance, in-house preparation is preferred. Preferred anode materials include copper and hard carbon electrodes, both of which are highly stable. The copper electrode is suitable for mechanism studies, while the hard carbon electrode is suitable for full-cell construction. The copper electrode is copper foil cut to size, with a purity exceeding 99.9% and a thickness of 5-20 μm. The hard carbon electrode is prepared by mixing hard carbon powder with polyvinylidene fluoride (PVDF) and conductive carbon black (super P) in a mass ratio of 85:8:7 in N-methylpyrrolidone (NMP). The mixture is then coated onto an aluminum foil current collector. The coated hard carbon electrode is then vacuum-dried at 110°C for 12 hours to obtain the hard carbon electrode.

[0037] Constructing a pretreatment battery system: The pretreatment battery system is used to pretreat the negative electrode material itself. The pretreatment battery system is a sodium-ion battery and can be a full cell or a half-cell. If it is a full cell, the negative electrode material to be treated is used as the negative electrode, a conventional sodium-ion battery positive electrode material is used as the positive electrode, and a predetermined sodium salt solution is used as the electrolyte to obtain the pretreated full cell. The sodium-ion battery positive electrode material can be a layered oxide positive electrode material, a polyanion positive electrode material, a Prussian blue analog positive electrode material, or an organic positive electrode material. If it is a half-cell, the negative electrode material to be treated is used as the working electrode, metallic sodium metal is used as the counter electrode and reference electrode, commercially available glass fiber is used as the battery separator, and a predetermined sodium salt solution is used as the electrolyte to assemble into a half-cell, such as a button cell or pouch cell. Regardless of whether the pretreatment battery system is a full cell or a half-cell, the electrolyte concentration is greater than that of the target battery system. The target battery system is a sodium-ion battery in which the pretreated negative electrode material will ultimately serve as the negative electrode. The electrolyte of the pretreatment battery system is the same as or different from the electrolyte of the target battery system, and is a mixture of sodium salt and organic solvent, and the sodium salt is selected from sodium hexafluorophosphate (NaPF6), sodium bis(fluorosulfonyl)imide (NaFSI), sodium nitrate (NaNO3), sodium perchlorate (NaClO4), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), sodium trifluoromethanesulfonate (NaOTf), sodium chloride (NaCl), sodium tetrafluoroborate (NaBF4), sodium methanesulfonate (NaCH3SO3), sodium trifluoromethanesulfonate (NaCF3SO3), sodium bis(oxalatoborate) (C4BNaO8), difluorooxalic acid At least one of sodium borate (C2BF2NaO4), sodium difluorophosphate (NaPO2F2), sodium difluorobis(oxalatophosphate) (NaDFBP), and sodium hexafluoroarsenate (NaAsF6) is used; the organic solvent is at least one of ethylene glycol dimethyl ether (DME), diethylene glycol dimethyl ether (DEGDME), tetraethylene glycol dimethyl ether (TEGDME), propylene carbonate (PC), diethyl carbonate (DEC), dioxolane (DOL), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), vinylene carbonate (VC), fluoroethylene carbonate (FEC), and tetrahydrofuran (THF). When constructing a pretreatment battery system, the preferred sodium salt is sodium bis(fluorosulfonyl)imide (NaFSI), and the preferred solvent is ethylene glycol dimethyl ether (DME). In an argon-protected glove box with a water and oxygen concentration of less than 0.01 ppm, the preferred sodium salt is dissolved in the preferred solvent and stirred until completely dissolved to obtain an electrolyte.

[0038] The pretreatment battery system is subjected to charge and discharge cycling: After the pretreatment battery system is constructed, an electrode-electrolyte interface is formed on the negative electrode material body through charge and discharge cycling. The charge-discharge current is less than 100μA or 1C, the voltage is (0.0002-2)V, and the number of charge and discharge cycles is at least 1 cycle, preferably 3 cycles. When the negative electrode material body is a copper electrode, a current of (5-100)μA is used in the voltage range of (0.0002-2)V for three cycles to form a thorough and stable electrode-electrolyte interface. A current of 5μA is preferred for more thorough and stable electrode-electrolyte interface formation. When the negative electrode material body is a hard carbon electrode, a current of (0.1-1)C is used in the voltage range of (0.0002-2)V for three cycles to form an electrode-electrolyte interface. Among them, a low current of 0.1C is preferred to form a more thorough and stable electrode-electrolyte interface.

[0039] Taking out the negative electrode material body: disassemble the pre-treated battery system, take out the negative electrode material body therein to obtain the pre-treated negative electrode material.

[0040] By using the negative electrode material obtained by the above method as the negative electrode to construct a sodium ion battery, preferably a commercial full battery, a battery system with excellent performance can be obtained. Example 1

[0041] A method for pretreating anode materials of sodium ion batteries comprises the following steps

[0042] (1) Obtaining the negative electrode material body: Cut the copper foil into discs with a diameter of 19 mm to obtain the negative electrode material body;

[0043] (2) Constructing a pretreatment battery system: a. Preparing an electrolyte: using sodium bis(fluorosulfonyl)imide as sodium salt and ethylene glycol dimethyl ether as solvent, the electrolyte was fully dissolved in an argon atmosphere with a water and oxygen value less than 0.01 ppm, and the concentration of sodium bis(fluorosulfonyl)imide was 4 mol / L; b. Using the negative electrode material obtained in step (1) as the working electrode, sodium metal as the counter electrode and reference electrode, and glass fiber as the battery separator, 150 μL of electrolyte was added to assemble a button battery;

[0044] (3) Performing charge and discharge cycles: The button cell is cycled three times at a low current of 5uA within the range of 0.0002-2V to form an electrode-electrolyte interface on the surface of the negative electrode material.

[0045] (4) Remove the negative electrode material. Example 2

[0046] A method for pretreating anode materials of sodium ion batteries comprises the following steps

[0047] (1) Obtaining the negative electrode material: Commercial-grade hard carbon powder is mixed with polyvinylidene fluoride (PVDF) and conductive carbon black (super P) in a mass ratio of 85:8:7 in N-methylpyrrolidone (NMP) and coated on a commercial aluminum foil current collector. The coated hard carbon electrode is vacuum-dried at 110 degrees Celsius for 12 hours to obtain a hard carbon electrode.

[0048] (2) Constructing a pretreatment battery system: a. Preparing an electrolyte: sodium bis(fluorosulfonyl)imide is used as the sodium salt and ethylene glycol dimethyl ether is used as the solvent. The electrolyte is fully dissolved in an argon atmosphere with a water and oxygen value less than 0.01 ppm. The concentration of sodium bis(fluorosulfonyl)imide is 4 mol / L. b. Using the negative electrode material obtained in step (1) as the working electrode, sodium metal as the counter electrode and reference electrode, and glass fiber as the battery separator, 150 μL of electrolyte is added to assemble a button battery. The sodium metal electrode is obtained by removing the outer oxide layer of sodium metal in a glove box protected by an argon atmosphere with a water and oxygen value less than 0.01 ppm.

[0049] (3) Performing charge and discharge cycles: The button cell is cycled three times at a low current of 0.1C within the range of 0.0002-2V to form an electrode-electrolyte interface on the surface of the negative electrode material.

[0050] (4) Remove the negative electrode material. Example 3

[0051] A method for pretreating anode materials of sodium ion batteries comprises the following steps

[0052] (1) Obtaining the negative electrode material body: Cut the copper foil into discs with a diameter of 19 mm to obtain the negative electrode material body;

[0053] (2) Constructing a pretreatment battery system: a. Preparing an electrolyte: using sodium bis(fluorosulfonyl)imide as sodium salt and ethylene glycol dimethyl ether as solvent, the electrolyte was fully dissolved in an argon atmosphere with a water and oxygen value less than 0.01 ppm, and the concentration of sodium bis(fluorosulfonyl)imide was 1 mol / L; b. Using the negative electrode material obtained in step (1) as the working electrode, sodium metal as the counter electrode and reference electrode, and glass fiber as the battery separator, 150 μL of electrolyte was added to assemble a button battery;

[0054] (3) Performing charge and discharge cycles: The button cell is cycled three times at a low current of 5uA within the range of 0.0002-2V to form an electrode-electrolyte interface on the surface of the negative electrode material.

[0055] (4) Remove the negative electrode material. Example 4

[0056] A method for pretreating anode materials of sodium ion batteries comprises the following steps

[0057] (1) Obtaining the negative electrode material: Commercial-grade hard carbon powder is mixed with polyvinylidene fluoride (PVDF) and conductive carbon black (super P) in a mass ratio of 85:8:7 in N-methylpyrrolidone (NMP) and coated on a commercial aluminum foil current collector. The coated hard carbon electrode is vacuum-dried at 110 degrees Celsius for 12 hours to obtain a hard carbon electrode.

[0058] (2) Constructing a pretreatment battery system: a. Preparing an electrolyte: using sodium bis(fluorosulfonyl)imide as sodium salt and ethylene glycol dimethyl ether as solvent, the electrolyte is fully dissolved under the protection of an argon atmosphere with a water and oxygen value less than 0.01 ppm, and the concentration of sodium bis(fluorosulfonyl)imide is 1 mol / L; b. Using the negative electrode material obtained in step (1) as the working electrode, sodium metal as the counter electrode and reference electrode, and glass fiber as the battery separator, 150 μL of electrolyte is added to assemble a button battery, and the sodium metal electrode is obtained by removing the outer oxide layer of sodium metal in a glove box protected by an argon atmosphere with a water and oxygen value less than 0.01 ppm;

[0059] (3) Performing charge and discharge cycles: The button cell is cycled three times at a low current of 0.1C within the range of 0.0002-2V to form an electrode-electrolyte interface on the surface of the negative electrode material.

[0060] (4) Remove the negative electrode material. Example 5

[0061] A method for pretreating anode materials of sodium ion batteries comprises the following steps

[0062] (1) Obtaining the negative electrode material body: Cut the copper foil into discs with a diameter of 19 mm to obtain the negative electrode material body;

[0063] (2) Constructing a pretreatment battery system: a. Preparing an electrolyte: using sodium bis(fluorosulfonyl)imide as sodium salt and ethylene glycol dimethyl ether as solvent, the electrolyte was fully dissolved in an argon atmosphere with a water and oxygen value less than 0.01 ppm, and the concentration of sodium bis(fluorosulfonyl)imide was 0.2 mol / L; b. Using the negative electrode material obtained in step (1) as the working electrode, sodium metal as the counter electrode and reference electrode, and glass fiber as the battery separator, 150 μL of electrolyte was added to assemble a button battery;

[0064] (3) Performing charge and discharge cycles: The button cell is cycled three times at a low current of 5uA within the range of 0.0002-2V to form an electrode-electrolyte interface on the surface of the negative electrode material.

[0065] (4) Remove the negative electrode material. Example 6

[0066] A method for pretreating anode materials of sodium ion batteries comprises the following steps

[0067] (1) Obtaining the negative electrode material: Commercial-grade hard carbon powder is mixed with polyvinylidene fluoride (PVDF) and conductive carbon black (super P) in a mass ratio of 85:8:7 in N-methylpyrrolidone (NMP) and coated on a commercial aluminum foil current collector. The coated hard carbon electrode is vacuum-dried at 110 degrees Celsius for 12 hours to obtain a hard carbon electrode.

[0068] (2) Constructing a pretreatment battery system: a. Preparing an electrolyte: sodium bis(fluorosulfonyl)imide is used as the sodium salt and ethylene glycol dimethyl ether is used as the solvent. The electrolyte is fully dissolved in an argon atmosphere with a water and oxygen value less than 0.01 ppm. The concentration of sodium bis(fluorosulfonyl)imide is 0.2 mol / L. b. Using the negative electrode material obtained in step (1) as the working electrode, sodium metal as the counter electrode and reference electrode, and glass fiber as the battery separator, 150 μL of electrolyte is added to assemble a button battery. The sodium metal electrode is obtained by removing the outer oxide layer of sodium metal in a glove box protected by an argon atmosphere with a water and oxygen value less than 0.01 ppm.

[0069] (3) Performing charge and discharge cycles: The button cell is cycled three times at a low current of 0.1C within the range of 0.0002-2V to form an electrode-electrolyte interface on the surface of the negative electrode material.

[0070] (4) Remove the negative electrode material. Example 7

[0071] The negative electrode material obtained in Example 2 was used as the negative electrode to assemble a full battery. The positive electrode material of the full battery was a manganese iron nickel based layered oxide, and the electrolyte was 1 M NaPF6in PC (propylene carbonate).

[0072] Commercially available manganese-iron-nickel-based layered oxide powder was mixed with polyvinylidene fluoride and conductive carbon black in a mass ratio of 85:8:7 in N-methylpyrrolidone and coated onto a 12mm diameter disc of aluminum foil. The coated hard carbon electrode was then vacuum-dried at 110°C for 12 hours to obtain a manganese-iron-nickel-based layered oxide electrode. Example 8

[0073] Full cell assembly: This differs from Example 7 in that the negative electrode material is a hard carbon anode obtained using the pretreatment method described in Example 4. The hard carbon anode is prepared by mixing commercial-grade hard carbon powder with polyvinylidene fluoride (PVDF) and conductive carbon black (super P) in a mass ratio of 85:8:7 in N-methylpyrrolidone (NMP). The mixture is then coated onto a commercial aluminum foil current collector. The coated hard carbon electrode is then vacuum-dried at 110°C for 12 hours to obtain the hard carbon electrode. Example 9

[0074] The difference from Example 4 is that the concentration of the electrolyte is 1.8 mol / L. Example 10

[0075] The only difference from Example 4 is that the concentration of the electrolyte is 3 mol / L. Example 11

[0076] The negative electrode material obtained by pretreatment in Example 9 is used to assemble a full battery, and the assembly method of the full battery is the same as that of Example 7. Example 12

[0077] A full battery is assembled using the negative electrode material obtained by the pretreatment method in Example 10, and the assembly method of the full battery is the same as that of Example 7.

[0078] The following shows the effect test of each embodiment

[0079] Effect test 1 - electrochemical capacity loss test

[0080] The negative electrode materials obtained in Example 1, Example 3, and Example 5 and metallic sodium were respectively assembled into button batteries. The electrolyte used was a solution of sodium bis(fluorosulfonyl)imide in ethylene glycol dimethyl ether. The concentration of the electrolyte in the button battery corresponding to Example 1 was 4 mol / L, the concentration of the electrolyte in the button battery corresponding to Example 3 was 1 mol / L, and the concentration of the electrolyte in the button battery corresponding to Example 5 was 0.2 mol / L. Except for the different electrolyte concentrations, the button batteries of each embodiment had the same structure. The battery was left standing for 50 hours - recycled to replenish the electrode electrolyte interface - left standing for 30 hours - recycled to replenish the electrode electrolyte interface - left standing for 15 hours, and the capacity loss change of each button battery was observed, as shown in FIG. Figure 2 As shown in the figure, it can be seen that the button cell corresponding to 4 mol / L has the smallest dissolution loss in each stage, especially after 50 hours of standing, the capacity loss is only 1.2μAh, which is much lower than the other two button cells, indicating that increasing the electrolyte concentration is beneficial to reducing the capacity loss of the negative electrode material after pretreatment. The capacity loss during the cumulative standing time of 95 hours (50 hours + 30 hours + 15 hours) is as follows Figure 3As shown, the degree of dissolution of the electrode-electrolyte interface can be reflected. It can be seen from the figure that the electrode-electrolyte interface formed under 4 mol / L electrolyte has the smallest cumulative capacity loss, which means that its dissolution degree is the smallest.

[0081] Effect Test 2 - Inductively Coupled Plasma Test

[0082] The negative electrode materials obtained in Example 1, Example 3 and Example 5 were respectively immersed in an equal volume of ethylene glycol dimethyl ether. After soaking for 50 hours, the sodium ion concentration in the ethylene glycol dimethyl ether was tested. The sodium ion concentration in the test solvent can characterize the degree of dissolution of the electrode electrolyte interface in the corresponding negative electrode material, and thus intuitively obtain its stability. The results are as follows Figure 4 As shown in the figure, we normalized the solubility of the negative electrode material corresponding to 4 mol / L in DME to 1 unit through ICP test. The sodium ion concentrations of the negative electrode material corresponding to 1 mol / L and the negative electrode material corresponding to 0.2 mol / L in the solvent are much greater than those of the negative electrode material corresponding to 4 mol / L, which are 3.38 times and 3.26 times respectively. It can be seen that the negative electrode material treated in high concentration electrolyte has high stability.

[0083] Effect test three - X-ray photoelectron spectroscopy (XPS) test

[0084] XPS was used to test the element distribution at the electrode-electrolyte interface of the negative electrode materials obtained in Example 1, Example 3 and Example 5. Figure 5 As shown in the figure, it can be seen that with the increase of concentration, the carbon element on the electrode electrolyte interface decreases, the organic component decreases, and the inorganic component increases. When the concentration reaches 4 mol / L, the carbon content decreases to 47%.

[0085] The negative electrode materials obtained in Example 1, Example 3 and Example 5 were immersed in a commercial sodium ion battery electrolyte, wherein the electrolyte was a 1 mol / L sodium bis(fluorosulfonyl)imide solution in ethylene glycol dimethyl ether, and immersed for 50 h. The changes in the element content of the negative electrode materials corresponding to each example before and after immersion were characterized by XPS, as shown in FIG. Figure 6 As shown, it can be seen that the solubility of each element in the solution of the negative electrode material corresponding to 1 mol / L is the largest, and the solubility of each element in the solution of the negative electrode material corresponding to 4 mol / L is the smallest. The dissolution of each element is effectively inhibited and is much smaller than that of other negative electrode materials.

[0086] The changes in the components of the electrode-electrolyte interface on the negative electrode material in each electrolyte are observed, forming the schematic diagrams shown in Figures 7 (a) to 7 (c). The electrode-electrolyte interface formed by the negative electrode material corresponding to 0.2 mol / L is shown in Figure 7 (a), forming an electrode-electrolyte interface rich in organic components. When immersed in the electrolyte (1 mol / L sodium bis(fluorosulfonyl)imide in ethylene glycol dimethyl ether solution), the organic components are dissolved; the electrode-electrolyte interface formed by the negative electrode material corresponding to 1 mol / L is shown in Figure 7 (b), The electrode-electrolyte interface formed by the negative electrode material corresponding to 4 mol / L is shown in Figure 7 (c), which forms an organic-inorganic mixed electrode-electrolyte interface. When immersed in the electrolyte (1 mol / L sodium bis(fluorosulfonyl)imide in ethylene glycol dimethyl ether solution), the organic component dissolves. The electrode-electrolyte interface formed by the negative electrode material corresponding to 4 mol / L is shown in Figure 7 (c), which forms an electrode-electrolyte interface rich in inorganic components. When immersed in the electrolyte (1 mol / L sodium bis(fluorosulfonyl)imide in ethylene glycol dimethyl ether solution), the entire interface is almost insoluble, the dissolution is effectively suppressed, and the interface stability is high.

[0087] Effect Test 4 - Coulombic Efficiency of Full Battery in the First Week

[0088] The full batteries prepared in Example 7 and Example 8 were subjected to charge and discharge cycles, and the charge and discharge curves and the first cycle coulombic efficiency were tested. Figure 8 As shown, it can be seen that the full battery (insoluble SEI) formed by assembling the negative electrode materials after pretreatment with high concentration electrolyte (4M) inhibited the excessive electrode-electrolyte interface growth and other side reactions in the first week, and achieved a high first-week coulombic efficiency of 90.29%. The full battery (control group SEI) formed by assembling the negative electrode materials pretreated with conventional concentration electrolyte (1M) could not inhibit the dissolution and growth of the electrode-electrolyte interface, and thus had a low first-week coulombic efficiency of only 80.69%.

[0089] Effect Test 5 - Capacity Retention Rate of Full Battery After 900 Cycles

[0090] The full batteries prepared in Example 7 and Example 8 were subjected to charge and discharge cycles for 900 cycles, and the changes in the specific capacity of the batteries during the cycles were recorded. Figure 9 As shown, the sodium ion full battery (insoluble SEI) assembled from the negative electrode materials pretreated with high-concentration electrolyte maintained a high capacity retention rate of 80.0% after 900 cycles, while the full battery (control group SEI) assembled from the negative electrode materials pretreated with conventional concentration electrolyte only maintained a lower level of 46.8%.

[0091] Effect Test 6 - Coulombic efficiency of full battery after 900 cycles

[0092] The full batteries prepared in Example 7 and Example 8 were subjected to charge and discharge cycles for 900 cycles, and the changes in the coulombic efficiency of the full batteries during the cycles were recorded. Figure 10As shown, it can be seen that the sodium ion full battery (insoluble SEI) formed by the negative electrode material pretreated with high concentration electrolyte achieved a high average coulombic efficiency of 99.95%, while the full battery (control group SEI) assembled by the negative electrode material pretreated with conventional concentration electrolyte had a lower average coulombic efficiency of only 99.02%.

[0093] The full cells of Examples 11 and 12 were subjected to charge and discharge cycles, and the changes in the specific capacity and coulombic efficiency of the cells during the cycles were recorded. Testing revealed that after 900 cycles, the capacity retention of the full cells of Examples 11 and 12 was significantly higher than that of the full cells corresponding to the negative electrode materials pretreated with a 1 mol / L electrolyte. The coulombic efficiency of the full cells obtained in Examples 11 and 12 was higher than that of the full cells corresponding to the negative electrode materials pretreated with a 1 mol / L electrolyte. After immersion in an electrolyte (a 1 mol / L solution of sodium bis(fluorosulfonyl)imide in ethylene glycol dimethyl ether), the entire interface remained virtually insoluble, effectively suppressing dissolution and demonstrating high interfacial stability.

[0094] From the above data, it can be seen that the pre-formed insoluble electrode-electrolyte interface on the negative electrode material helps to improve the stability of the interface and improve the coulombic efficiency of the battery. When the organic component content on the electrode-electrolyte interface of the negative electrode material is less than the inorganic component content or the carbon content is less than 50% or the interface component is immersed in an organic solvent for 50 hours, the component loss rate is less than 15%, the assembled full battery effect is better.

[0095] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for pretreating a negative electrode material for a sodium ion battery, characterized in that: The following steps are involved: Obtaining the negative electrode material body; Constructing a pretreatment battery system: constructing a pretreatment battery system with the negative electrode material as the working electrode, the concentration of the electrolyte in the pretreatment battery system is greater than the concentration of the electrolyte in the target battery system, the pretreatment battery system is a battery system in which the negative electrode material is pretreated, the pretreatment battery system is a sodium ion battery, and the target battery system is a battery system in which the pretreated negative electrode material is used as the negative electrode; Performing a charge-discharge cycle on the pre-treated battery system: performing a charge-discharge cycle on the constructed pre-treated battery system to form an electrode-electrolyte interface on the negative electrode material body, wherein the concentration of the electrolyte in the pre-treated battery system is higher than 1.8 mol / L; The negative electrode material body is taken out to obtain the pretreated negative electrode material.

2. The method for pretreating a negative electrode material for a sodium ion battery according to claim 1, wherein: The pretreatment battery system is constructed in one of the following ways: ——Use the negative electrode material as the working electrode and sodium metal as the counter electrode to construct a half-cell system; ——Build a full battery system with the negative electrode material as the negative electrode and the sodium battery positive electrode material as the positive electrode.

3. The method for pretreating a negative electrode material for a sodium ion battery according to claim 1, wherein: The concentration of the electrolyte in the pretreatment battery system is higher than 4 mol / L.

4. The method for pretreating a negative electrode material for a sodium ion battery according to claim 1, wherein: The negative electrode material body is a hard carbon electrode material or a copper electrode material; When the hard carbon electrode material is used as the negative electrode material, the sodium ion battery system is subjected to a charge-discharge cycle at a current of (0.1-1) C and a voltage of (0.0002-2) V; When the copper electrode material is used as the negative electrode material, the sodium ion battery system performs charge and discharge cycles at a current of (5-100) μA and a voltage of (0.0002-2) V.

5. The method for pretreating a negative electrode material for a sodium ion battery according to claim 1 or 4, wherein: During the charge and discharge cycle of the pretreatment battery system, the number of cycles is at least 1.

6. The method for pretreating a negative electrode material for a sodium ion battery according to claim 5, wherein: During the charge and discharge cycle of the pre-treated battery system, the number of cycles is more than 3.

7. The method for pretreating a negative electrode material for a sodium ion battery according to claim 1, wherein: The electrolyte components in the pretreatment battery system are the same as or different from those in the target battery system. The electrolyte of the pretreatment battery system and / or the target battery system comprises a sodium salt and a solvent, wherein the solvent is an organic solvent, and the sodium salt and the solvent satisfy at least one of the following conditions: The sodium salt is a combination of one or more of sodium hexafluorophosphate, sodium bis(fluorosulfonylimide), sodium nitrate, sodium perchlorate, sodium bis(trifluoromethylsulfonylimide), sodium trifluoromethanesulfonate, sodium chloride, sodium tetrafluoroborate, sodium methanesulfonate, sodium trifluoromethanesulfonate, sodium bis(oxalatoborate), sodium difluorooxalatoborate, sodium difluorophosphate, sodium difluorobis(oxalatophosphate), and sodium hexafluoroarsenate; The solvent is a combination of one or more of ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, propylene carbonate, diethyl carbonate, dioxolane, ethyl methyl carbonate, dimethyl carbonate, vinylene carbonate, fluoroethylene carbonate, and tetrahydrofuran.

8. The method for pretreating a negative electrode material for a sodium ion battery according to claim 7, wherein: The sodium salt in the pretreatment battery system is sodium bis(fluorosulfonyl)imide, and the solvent is ethylene glycol dimethyl ether.

9. The method for pretreating a negative electrode material for a sodium ion battery according to claim 4, wherein: The preparation method of the hard carbon electrode comprises: uniformly mixing hard carbon powder, a binder and a conductive agent in a dispersant according to a set ratio, and coating the mixture on a conductive current collector; and vacuum drying the coated hard carbon electrode to obtain the hard carbon electrode.

10. The method for pretreating a negative electrode material for a sodium ion battery according to claim 9, wherein: The binder is polyvinylidene fluoride, the conductive agent is conductive carbon black, and the dispersant is N-methylpyrrolidone.

11. The method for pretreating a negative electrode material for a sodium ion battery according to claim 1, wherein: The atomic percentage of carbon in the components at the electrode electrolyte interface is less than 50%.

12. A sodium ion battery negative electrode material obtained by the pretreatment method according to claim 1.

13. A sodium ion battery, characterized in that: The sodium ion battery negative electrode material according to claim 12 is used as the negative electrode.

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