Sodium-supplementing negative electrode active material and preparation method thereof, negative electrode, sodium-ion battery and preparation method of sodium-supplementing negative electrode active material

Abstract

The invention relates to the technical field of energy storage batteries, in particular to a sodium-supplementing negative electrode active material and a preparation method thereof, a negative electrode, a sodium ion battery and a preparation method. The sodium-supplementing negative electrode active material comprises a hard carbon material, a sodium element deposited on the hard carbon material and a protective film formed on the hard carbon material. The preparation method of the sodium-supplementing negative electrode active material is simple in process and high in efficiency, and the sodium supplementing amount and the sodium supplementing uniformity of the negative electrode active material can be accurately controlled by changing the insertion potential and the separation potential and adjusting the current density, the bath temperature and the insertion and separation times; therefore, the first coulombic efficiency and the cycling stability of the whole sodium ion battery are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of energy storage batteries, and in particular to a sodium-supplementing negative electrode active material and a preparation method thereof, a negative electrode, and a sodium ion battery and a preparation method thereof. Background Art

[0002] With the continuous expansion of the new energy vehicle market and the rapid development of the electronics industry, the demand for lithium-ion batteries is rapidly increasing. However, given the limited and high cost of lithium resources, the development of new, low-cost, non-lithium energy storage devices with abundant raw materials is a current research priority. Sodium-ion batteries are entering the mainstream. In terms of reserves, sodium accounts for approximately 2.74% of Earth's crustal abundance, while lithium accounts for only 0.0065%. Globally, 70% of lithium resources are located in a few countries and regions, such as South America, while sodium resources are distributed globally, making them an ideal alternative to lithium batteries.

[0003] The working principle of sodium-ion batteries is similar to that of lithium-ion batteries. It uses the reversible insertion and removal of sodium ions between the positive and negative electrodes to achieve energy storage and release. Therefore, this battery also has the problem of low initial coulombic efficiency. This is mainly because the sodium ions released from the positive electrode during the first cycle of charging of the sodium-ion battery will react at the negative electrode to form an SEI film or other side reactions. The SEI film will repeatedly decompose during subsequent cycles, resulting in the loss of active sodium ions. When the sodium-ion battery is discharged, the same amount of sodium ions cannot be released from the negative electrode to return to the positive electrode, and the performance of the material cannot be fully utilized, resulting in low battery capacity. Therefore, it is necessary to supplement sodium at the negative electrode to improve the battery's coulombic efficiency and cycle stability.

[0004] At present, in order to solve the problem of low initial efficiency of sodium-ion batteries, researchers have proposed many solutions. Chinese invention patent CN110783525A discloses positive electrode additives for sodium-ion batteries, battery positive electrodes, sodium-ion batteries and applications, which can reduce the consumption of sodium ions and improve the energy density of sodium-ion batteries. By adding additives to the sodium-ion positive electrode material, this process method is simple, but these positive electrode additives are very active and easily oxidized to lose their activity. After being assembled into battery cells, they are prone to gasification during the cycle, leading to the problem of battery cell swelling. Chinese invention patent CN108878780A discloses that the present invention discloses a method for replenishing sodium in the negative electrode of a sodium-ion battery and a sodium-ion battery, comprising: in an inert atmosphere, melting solid metallic sodium at a certain temperature to obtain liquid metallic sodium; adding the liquid metallic sodium to the surface of the negative electrode sheet so that the liquid metallic sodium penetrates into the gaps between the negative electrode materials of the negative electrode sheet, drying the negative electrode sheet and assembling it into a battery. This method also has problems. The activity of elemental sodium is high, the production conditions are harsh, it needs to be carried out in an inert atmosphere, and the safety is low, which affects the subsequent battery assembly process. Chinese invention patent CN113644271A discloses a method for preparing a sodium-ion battery negative electrode sodium supplement additive and a negative electrode material. The raw materials for preparing the additive include a composite of carbon nanotubes and metallic sodium, and the composite of carbon nanotubes and metallic sodium is mixed with a hard carbon material to obtain a solid negative electrode material. This method has the disadvantage of a more complicated process, and an organic solvent is introduced into the negative electrode. During the sodium ion battery manufacturing stage, organic matter may remain in the negative electrode, thereby deteriorating the performance of the sodium ion battery. Summary of the Invention

[0005] The purpose of the present invention is to overcome the deficiencies and defects of the prior art and to provide a sodium-supplementing negative electrode active material and a preparation method thereof, a negative electrode, and a sodium ion battery and a preparation method thereof.

[0006] In order to achieve the above objectives, this application adopts the following solutions:

[0007] A sodium-supplementing negative electrode active material comprises a hard carbon material, sodium elements deposited on the hard carbon material, and a protective film formed on the hard carbon material.

[0008] The present invention also includes a method for preparing the sodium-supplementing negative electrode active material, comprising the following steps: in an electrolytic cell, using a sodium source as a positive electrode, a hard carbon material as a negative electrode, and sodium salt and a solvent as a deposition liquid to electro-deposit the hard carbon material onto the sodium-supplementing negative electrode.

[0009] The process parameters of electrodeposition are: insertion potential is -0.2V to 0V, and separation potential is 0.3V to 0.8V; the separation potential is higher than the deposition potential; the current density is 0.1 to 2 mA / cm 2; The number of cycles is 1 to 5 constant current charge and discharge cycles; the temperature is 40 to 60°C.

[0010] The process parameters of electrodeposition are: insertion potential of -0.1V to 0V, separation potential of 0.4V to 0.6V; separation potential is higher than deposition potential; current density of 0.5 to 1.5 mA / cm 2 ; The number of cycles is 1 to 3 constant current charge and discharge cycles; the temperature is 50℃.

[0011] The sodium salt in the sedimentation liquid is sodium hexafluorophosphate NaPF6 and / or sodium difluorooxalatoborate NaDFOB, preferably a mixture of sodium hexafluorophosphate NaPF6 and sodium difluorooxalatoborate NaDFOB, with a molar ratio of 1:0.3;

[0012] Preferably, the concentration of the sodium salt is 0.8 to 1.5 mol / kg; preferably 1.3 mol / kg.

[0013] The solvent in the deposition liquid is ethylene carbonate EC and / or propylene carbonate PC; preferably, it is a mixture of ethylene carbonate EC and propylene carbonate PC, with a volume ratio of 1:1; more preferably, 1-5wt% fluoroethylene carbonate FEC is added to the deposition liquid as an additive, and more preferably 3wt% fluoroethylene carbonate FEC is added as an additive.

[0014] The present invention also includes a negative electrode, comprising the sodium-supplementing negative electrode active material, a negative electrode conductive agent, and a negative electrode binder;

[0015] Preferably, the negative electrode conductive agent is conductive carbon black;

[0016] Preferably, the negative electrode binder is PVDF;

[0017] Preferably, the mass ratio of the sodium-supplementing negative electrode active material, the negative electrode conductive agent and the negative electrode binder is (90-98): (1-5): (1-5); preferably 94:2:4;

[0018] Preferably, the negative electrode is prepared by the following method: mixing the sodium-supplemented negative electrode active material, the negative electrode conductive agent, the negative electrode binder, and the NMP solvent to prepare a slurry, coating the prepared slurry on the copper foil according to the designed coating amount, and obtaining the negative electrode through processes such as drying, rolling, cutting, and tab welding.

[0019] The present invention also includes a sodium ion battery, comprising a positive electrode, the negative electrode and a separator.

[0020] The positive electrode comprises a positive electrode active material, a positive electrode conductive agent and a positive electrode binder;

[0021] Preferably, the positive electrode conductive agent is conductive carbon black and / or conductive carbon nanotubes; preferably, it is a mixture of conductive carbon black and conductive carbon nanotubes, with a mass ratio of the two being 2:1;

[0022] Preferably, the positive electrode active material is one or more of sodium vanadium phosphate, sodium vanadium fluorophosphate, sodium cobaltate, sodium manganate, sodium nickelate, layered oxides and Prussian blue materials; preferably layered oxides;

[0023] Preferably, the positive electrode binder is PVDF;

[0024] Preferably, the mass ratio of the positive electrode active material, the positive electrode conductive agent and the positive electrode binder is (90-98): (1-5): (1-5); preferably 94.6:3:2.4;

[0025] Preferably, the positive electrode is prepared by the following method: the positive electrode active material, positive electrode conductive agent, positive electrode binder, and NMP solvent are mixed to prepare slurry, the prepared slurry is coated on aluminum foil according to the designed coating amount, and the negative electrode is obtained through processes such as drying, rolling, cutting, and tab welding.

[0026] The present invention also includes a preparation method of the sodium ion battery, comprising the following steps: forming a positive electrode, a separator, and a negative electrode into an electrode group through a winding process, performing X-ray inspection, shelling, bottom welding, closing, grooving, dust removal, electrolyte injection, cover welding, sealing, etc. to obtain a sodium ion secondary battery; after the battery is left to stand for a period of time at room temperature, it enters pre-formation, primary standing, main formation, secondary standing, high-temperature aging, and post-treatment.

[0027] Compared with the prior art, the implementation scheme of this application:

[0028] The sodium supplementation method provided by the present invention obtains a sodium-supplemented negative electrode active material, which can overcome the shortcomings of traditional sodium supplementation technologies, such as low efficiency and complex procedures. In addition, some methods introduce inactive impurities, which reduces the overall energy density of the sodium ion battery and thus deteriorates the performance of the sodium ion battery.

[0029] This patent provides a method for preparing sodium-supplemented negative electrode active materials. The process is simple and efficient. By changing the insertion potential and release potential, adjusting the current density, bath temperature, and the number of insertions and releases, the amount of sodium replenished in the negative electrode active material and the uniformity of sodium replenishment can be controlled more accurately, thereby improving the initial coulombic efficiency and cycle stability of the entire sodium-ion battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a cycle capacity retention curve diagram of Example 5 of the present invention and a comparative example;

[0031] Figure 2Schematic diagram of the structure of the electrolytic cell of the present invention. DETAILED DESCRIPTION

[0032] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0033] The preparation method of a sodium-supplementing negative electrode active material comprises the following steps: in an electrolytic cell, using a sodium source as the positive electrode, a hard carbon material as the negative electrode, and a sodium salt and a solvent as a deposition solution to electrodeposit the hard carbon material onto the sodium-supplementing negative electrode. The electrodeposition process parameters are: an insertion potential of -0.2V to 0V, a desorption potential of 0.3V to 0.8V; the desorption potential is higher than the deposition potential to ensure stable sodium ion desorption; and a current density of 0.1 to 2 mA / cm 2 The number of cycles is 1 to 5 constant current charge and discharge cycles to ensure uniform deposition of sodium and the formation of a stable SEI film; the temperature is 40 to 60°C. Within this temperature range, the diffusion rate of sodium ions and the stability of the electrolyte reach a balance.

[0034] Figure 2 The structure of the electrolytic cell 1 used in this application is shown, which includes a cell body 8, a positive electrode 2 disposed in the cell body, a negative electrode powder container 6, and a separator 5 disposed between the positive electrode and the negative electrode powder container; an external power supply 7 is disposed between the positive electrode and the negative electrode; a sedimentation liquid 4 is disposed in the cell body; and the negative electrode powder container 6 is provided with negative electrode powder 3.

[0035] It has been verified that as the preferred form, the process parameters of electrodeposition are: insertion potential is -0.1V~0V, separation potential is 0.4V~0.6V; separation potential is higher than deposition potential; current density is 0.5~1.5mA / cm 2 ; The number of cycles is 1 to 3 constant current charge and discharge cycles; the temperature is 50℃.

[0036] The sodium salt in the sedimentation liquid is sodium hexafluorophosphate NaPF6 and / or sodium difluorooxalatoborate NaDFOB, preferably a mixture of sodium hexafluorophosphate NaPF6 and sodium difluorooxalatoborate NaDFOB, with a molar ratio of 1:0.3;

[0037] Preferably, the concentration of the sodium salt is 0.8 to 1.5 mol / kg; preferably 1.3 mol / kg.

[0038] The solvent in the deposition liquid is ethylene carbonate EC and / or propylene carbonate PC; preferably, it is a mixture of ethylene carbonate EC and propylene carbonate PC, with a volume ratio of 1:1; more preferably, 1-5wt% fluoroethylene carbonate FEC is added to the deposition liquid as an additive, more preferably 1wt% fluoroethylene carbonate FEC is added as an additive.

[0039] The following describes the details with reference to specific embodiments.

[0040] Example 1

[0041] The sodium salt in the deposition solution is sodium hexafluorophosphate (NaPF6) at a concentration of 1.3 mol / kg. The solvent in the deposition solution is a mixture of ethylene carbonate (EC) and propylene carbonate (PC) at a volume ratio of 1:1. 1 wt% fluoroethylene carbonate (FEC) is added as an additive.

[0042] In this embodiment, the positive electrode sodium source is selected from metallic sodium foil (purity ≥ 99.9%, thickness 0.3 mm), the negative electrode powder is selected from commercial hard carbon powder, and the separator is selected from PVDF double-sided glue-coated separator;

[0043] The process parameters of electrodeposition are: insertion potential (sodium deposition) is -0.1 V, and separation potential (sodium dissolution) is 0.6 V; the separation potential is higher than the deposition potential; the current density is 1 mA / cm 2 ; The number of cycles is 3 constant current charge and discharge cycles; the temperature is 50°C, and the time control is: single cycle time is 5 hours (total processing time ≤ 24 hours).

[0044] Example 2

[0045] The only difference between Example 2 and Example 1 is that the current density in the electrodeposition process parameters is adjusted to 0.5 mA / cm 2 .

[0046] Example 3

[0047] The only difference between Example 3 and Example 1 is that the current density in the electrodeposition process parameters is adjusted to 2 mA / cm 2 .

[0048] Comparing the three examples, the morphologies of sodium deposition are different due to different current densities. At 0.5 mA / cm2, it is a dense layered structure, and at 1 mA / cm 2 , which is relatively 0.5mA / cm 2 , the structure is looser and the current density is higher than 2mA / cm 2 Nanodendrites will be generated locally.

[0049] Example 4

[0050] The only difference between Example 4 and Example 1 is that the amount of fluoroethylene carbonate (FEC) added to the deposition solution is adjusted to 3wt%. The results show that when the FEC content increases to 3%, the NaF content in the SEI film formed on the negative electrode surface increases and the film layer becomes denser, which is beneficial to improving the cycle stability of the battery.

[0051] Example 5

[0052] Example 5 differs from Example 1 only in the sodium salt used in the deposition solution: sodium hexafluorophosphate (NaPF6) at a concentration of 1.0 mol / kg. The auxiliary salt, sodium difluorooxalatoborate (NaDFOB) at a concentration of 0.3 mol / kg, was used. Results show that the disodium salt system forms a trapezoidal SEI film on the negative electrode surface (with an inner layer primarily composed of NaF / Na2O and an outer layer containing BO compounds). This system significantly improves ionic conductivity and reduces gas production.

[0053] The sodium-supplementing negative electrode active materials obtained in Examples 1-5 are used to prepare negative electrodes and sodium ion batteries.

[0054] A sodium ion battery is prepared by the following method:

[0055] 1) Positive Electrode Preparation: A layered oxide was selected as the positive electrode active material, with the mass ratio of positive electrode active material: conductive carbon black: conductive carbon nanotubes: binder = 94.6:2:1:2.4. The layered oxide material, polyvinylidene fluoride (PVDF) slurry, conductive carbon black, and conductive carbon nanotube slurry were mixed in the above proportions with NMP solution and stirred to form a mixed slurry. The resulting slurry was cast on 12μm aluminum foil and dried, rolled, cut, and then tab welded to produce the positive electrode sheet.

[0056] 2) Negative Electrode Preparation: The sodium-supplementing negative electrode active materials prepared in Examples 1-5 were washed three times with dimethyl carbonate (DMC) to remove residual electrolyte. Drying: Vacuum drying (80°C, 10 hours) was performed to a moisture content of ≤50 ppm. Treated sodium-supplementing negative electrode active materials were obtained. Untreated hard carbon material was used as the negative electrode active material as Comparative Example 1.

[0057] The mass percentage of the negative electrode is: sodium-supplemented negative electrode active material: conductive carbon black: binder PVDF = 94:2:4. A slurry of the sodium-supplemented hard carbon negative electrode material, conductive carbon black, binder PVDF, and NMP solvent is mixed to form a slurry. This slurry is then coated onto 6μm copper foil according to the designed coating amount. The negative electrode sheet is then produced through drying, rolling, cutting, and tab welding.

[0058] 3) Assembly of sodium-ion secondary batteries: The positive electrode, separator, and negative electrode are wound into an electrode group, which is then subjected to X-ray inspection, shell insertion, bottom welding, closing, grooving, dust removal, electrolyte injection, cap welding, and sealing to obtain a sodium-ion secondary battery. After the battery is left to stand for a period of time at room temperature, it enters the pre-formation, primary standing, main formation, secondary standing, high-temperature aging, and post-processing steps.

[0059] The obtained sodium ion battery was subjected to performance testing, and the results are shown in Table 1.

[0060] Table 1

[0061] project Example 1 Example 2 Example 3 Example 4 Example 5 Comparative Example 1 Number of cycles 3 3 3 3 4 —— First coulombic efficiency 92％ 94％ 88％ 93％ 95％ 85％ SEI film characteristics Dense NaF-based layered structure local dendrites High NaF content Trapezoidal composite membrane Porous loose structure Gas production Low Very low Higher Low Very low high

[0062] Figure 1 The following is a graph showing the cycle capacity retention rate of Example 5 and the comparative example. The charge rate is 0.5C, the discharge rate is 1C, the charge and discharge cut-off voltage is 1.5V to 3.9V, and the capacity is recalibrated every 50 cycles. The cycling data shows that the cycle capacity retention rate of the battery obtained with the negative electrode pre-supplemented with sodium is significantly better than that of the negative electrode battery without sodium supplementation. The reason for this high cycle capacity retention rate is likely to be that the stable sodium supplementation of the negative electrode active material particles and the formation of a dense film on the material surface improve both the initial efficiency and the cycling characteristics.

[0063] The basic principles, main features and advantages of the present invention are shown and described above. It is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments and that the present invention can be implemented in other specific forms without departing from the spirit or basic features of the present invention.

[0064] The embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being defined by the appended claims rather than the foregoing description, and all changes that come within the meaning and range of equivalents of the claims are therefore intended to be embraced therein.

[0065] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A sodium-supplementing negative electrode active material, characterized in that: The invention comprises a hard carbon material, a sodium element deposited on the hard carbon material and a protective film formed on the hard carbon material.

2. A method for preparing the sodium-supplementing negative electrode active material according to claim 1, characterized in that: The method comprises the following steps: in an electrolytic cell, using a sodium source as a positive electrode, a hard carbon material as a negative electrode, and sodium salt and a solvent as a deposition liquid to electro-deposit the hard carbon material onto the sodium-supplemented negative electrode.

3. The method for preparing the sodium-supplementing negative electrode active material according to claim 2, wherein: The process parameters of electrodeposition are: insertion potential is -0.2V to 0V, and separation potential is 0.3V to 0.8V; the separation potential is higher than the deposition potential; the current density is 0.1 to 2 mA / cm 2 ; The number of cycles is 1 to 5 constant current charge and discharge cycles; the temperature is 40 to 60°C.

4. The method for preparing a sodium-supplementing negative electrode active material according to claim 2, wherein: The process parameters of electrodeposition are: insertion potential of -0.1V to 0V, separation potential of 0.4V to 0.6V; separation potential is higher than deposition potential; current density of 0.5 to 1.5 mA / cm 2 ; The number of cycles is 1 to 3 constant current charge and discharge cycles; the temperature is 50℃.

5. The method for preparing the sodium-supplementing negative electrode active material according to claim 2, wherein: The sodium salt in the sedimentation liquid is sodium hexafluorophosphate NaPF6 and / or sodium difluorooxalatoborate NaDFOB, preferably a mixture of sodium hexafluorophosphate NaPF6 and sodium difluorooxalatoborate NaDFOB, with a molar ratio of 1:0.3; preferably, the concentration of the sodium salt is 0.8-1.5 mol / kg; preferably 1.3 mol / kg.

6. The method for preparing a sodium-supplementing negative electrode active material according to claim 2, wherein: The solvent in the deposition liquid is ethylene carbonate EC and / or propylene carbonate PC; preferably, it is a mixture of ethylene carbonate EC and propylene carbonate PC, with a volume ratio of 1:1; more preferably, 1-5wt% fluoroethylene carbonate FEC is added to the deposition liquid as an additive, and more preferably 3wt% fluoroethylene carbonate FEC is added as an additive.

7. A negative electrode, characterized in that The invention comprises the sodium-supplementing negative electrode active material according to claim 1, a negative electrode conductive agent and a negative electrode binder; Preferably, the negative electrode conductive agent is conductive carbon black; Preferably, the negative electrode binder is PVDF; Preferably, the mass ratio of the sodium-supplementing negative electrode active material, the negative electrode conductive agent and the negative electrode binder is (90-98): (1-5): (1-5); preferably 94:2:4; Preferably, the negative electrode is prepared by the following method: mixing the sodium-supplemented negative electrode active material, the negative electrode conductive agent, the negative electrode binder, and the NMP solvent to prepare a slurry, coating the prepared slurry on the copper foil according to the designed coating amount, and obtaining the negative electrode through processes such as drying, rolling, cutting, and tab welding.

8. A sodium ion battery, characterized in that: The invention comprises a positive electrode, the negative electrode according to claim 7 and a separator.

9. The sodium ion battery according to claim 8, characterized in that The positive electrode comprises a positive electrode active material, a positive electrode conductive agent and a positive electrode binder; Preferably, the positive electrode conductive agent is conductive carbon black and / or conductive carbon nanotubes; preferably, it is a mixture of conductive carbon black and conductive carbon nanotubes, with a mass ratio of the two being 2:1; Preferably, the positive electrode active material is one or more of sodium vanadium phosphate, sodium vanadium fluorophosphate, sodium cobaltate, sodium manganate, sodium nickelate, layered oxides and Prussian blue materials; preferably layered oxides; Preferably, the positive electrode binder is PVDF; Preferably, the mass ratio of the positive electrode active material, the positive electrode conductive agent and the positive electrode binder is (90-98): (1-5): (1-5); preferably 94.6:3:2.4; Preferably, the positive electrode is prepared by the following method: the positive electrode active material, positive electrode conductive agent, positive electrode binder, and NMP solvent are mixed to prepare slurry, the prepared slurry is coated on aluminum foil according to the designed coating amount, and the negative electrode is obtained through processes such as drying, rolling, cutting, and tab welding.

10. A method for preparing a sodium ion battery according to claim 8 or 9, characterized in that: The method comprises the following steps: forming a positive electrode, a separator and a negative electrode into an electrode group through a winding process, performing X-ray inspection, shelling, bottom welding, closing, grooving, dust removal, electrolyte injection, cover welding and sealing to obtain a sodium ion secondary battery; and after the battery is left to stand for a period of time at room temperature, entering pre-formation, primary standing, main formation, secondary standing, high-temperature aging and post-processing.

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