Low self-discharge sodium ion battery

Abstract

The invention discloses a low self-discharge sodium ion battery, which comprises a positive electrode, a negative electrode and an electrolyte, an active material of the positive electrode is sodium ferric sulfate coated with titanium sodium phosphate, an active material of the negative electrode is titanium sodium phosphate, the electrolyte comprises a sodium salt, an organic solvent and an additive, and the additive is tris (trimethylsilyl) phosphate and sodium difluorophosphate. According to the invention, a sodium ferric sulfate material is used as a positive electrode active material, and the stability of the positive electrode is improved and the energy density of the battery is improved through the synergistic protection effect of the coated titanium sodium phosphate and the electrolyte additive; a negative electrode active material adopts a polyanion material sodium titanium phosphate with a highly stable lattice structure, and the potential of valence change of Ti ions is far higher than that of Na ions, so that the negative electrode does not generate sodium dendrites, the problem of interface side reaction with electrolyte does not exist, sodium ions are not consumed, and the service life of the battery is prolonged. And the capacity retention ratio, the cycling stability and the safety of the battery system are further ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium-ion batteries, and particularly to a low self-discharge sodium-ion battery. Background Art

[0002] Sodium-ion batteries have advantages such as low-temperature safety, fast charge and discharge, fast startup, and zero-power storage, and can be widely used in fields such as backup power supplies. However, under long-term floating charge conditions, sodium-ion batteries will have the following problems: positive electrode materials such as layered oxides and polyanion compounds may experience problems such as transition metal ion dissolution and structural collapse due to the continuous high-voltage environment; at the same time, side reactions continuously occur between the electrolyte and the surface of the negative electrode using carbon materials as active materials, resulting in the continuous growth of the SEI (solid electrolyte interface) film, irreversibly consuming sodium ions in the electrolyte and losing battery capacity; in addition, the decomposition of the electrolyte is accelerated under high-voltage conditions; the tiny current causes uneven deposition of sodium ions on the surface of the carbon negative electrode, forming sodium metal dendrites.

[0003] The above problems are mainly caused by electrochemical reactions in the battery system, unstable crystal structures of active materials, and the breakage and reconstruction of the SEI film. To avoid side reactions during long-term floating charge, a new battery system needs to be developed. Among them, the active material itself needs to have a stable lattice structure, and there is no generation and reconstruction of the SEI film on the negative electrode. Summary of the Invention

[0004] In view of the deficiencies in the prior art, the present invention provides a low self-discharge sodium-ion battery. The positive electrode active material selects sodium iron sulfate with a high voltage as the positive electrode active material. Through the synergistic protection of coated sodium titanium phosphate and electrolyte additives, the stability of the positive electrode is increased, and at the same time, the energy density of the battery is improved; the negative electrode active material uses sodium titanium phosphate, a polyanion material with a highly stable lattice structure. Since the potential for the valence change of Ti ions is much higher than that of Na ions, there will be no generation of sodium dendrites on the negative electrode, and there is no problem of interfacial side reactions with the electrolyte, and thus sodium ions will not be consumed, further ensuring the battery system capacity retention rate, cycle stability, and safety.

[0005] To solve the above technical problems, the present invention provides a low self-discharge sodium-ion battery, including a positive electrode, a negative electrode, and an electrolyte;

[0006] The active material of the positive electrode is sodium iron sulfate coated with sodium titanium phosphate, and the active material of the negative electrode is sodium titanium phosphate;

[0007] The electrolyte includes a sodium salt, an organic solvent, and an additive, and the additive is tris(trimethylsilyl) phosphate (TMSP) and sodium difluorophosphate (NaDFP).

[0008] The present invention uses a sodium iron sulfate material with a high voltage as the positive electrode active material, and coats the surface of sodium iron sulfate with sodium titanium phosphate to increase its structural stability; at the same time, the coating material sodium titanium phosphate also synergistically acts with the additives in the electrolyte. The phosphate groups of the coating material sodium titanium phosphate react with tris(trimethylsilyl) phosphate and sodium difluorophosphate to form a solid electrolyte membrane. Thus, the solid electrolyte membrane and the sodium titanium phosphate coating layer provide double protection for the positive electrode, greatly improving the structural stability of the positive electrode.

[0009] Specifically, TMSP reacts with the phosphate groups of sodium titanium phosphate coated on the surface of sodium iron sulfate to generate a silicon-based phosphate solid electrolyte membrane, which can effectively inhibit the dissolution of the positive electrode; NaDFP reacts with the phosphate groups of sodium titanium phosphate coated on the surface of sodium iron sulfate to form a fluorophosphate solid electrolyte membrane, which has certain ionic conductivity and chemical stability, and will not reduce the electrochemical performance of the positive electrode material while forming protection, ensuring that the material is suitable for long-term floating charge; the solid electrolyte membrane formed by the reaction of the two additives with sodium titanium phosphate and the sodium titanium phosphate coating layer jointly provide double protection for the positive electrode. Among them, the phosphate ester group (P-O-Si bond) in TMSP undergoes hydrolysis or transesterification reaction with PO4 3- in the electrolyte to generate a cross-linked phosphate network, and the reaction formula is: PO4 3- +TMSP→Na x PO y -Si-O-polymer membrane, and the generated silicon-oxygen-phosphate composite protective film has high mechanical strength and density, which can effectively inhibit the dissolution of the positive electrode. NaDFP is oxidized and decomposed on the surface of the positive electrode to release PO2F2 - ions, which combine with PO4 3- through P-O-P bonds to form sodium fluorophosphate (Na3PO3F2) or Na x (PO4) y F z complex, and the reaction formula is: PO4 3- +PO2F2 - →Na3PO3F2 / Na x (PO4) y F z , and the fluorophosphate membrane has both ionic conductivity and chemical stability and is suitable for long-term floating charge.

[0010] For the negative electrode active material of the present invention, sodium titanium phosphate, a polyanion material with a highly stable crystal lattice structure, is selected. Among them, the potential at which the Ti ions undergo valence change is much higher than that of the Na ions. Therefore, sodium dendrites will not be generated in the negative electrode sodium titanium phosphate, and the sodium ions of the negative electrode active material will not be consumed. At the same time, it will not undergo an interfacial reaction with the electrolyte, and there is no process of SEI film formation - dissolution - reconstruction, and the sodium ions of the electrolyte will not be consumed. In this way, the capacity retention rate of the battery can be effectively improved, and the cycle stability and safety of the battery system can be ensured.

[0011] The battery system of the present invention is stable. There is no process of dendrite formation - dissolution - reconstruction in both the positive electrode and the negative electrode, and the active sodium ions will not be consumed. When floating charging for a long time, the current is close to zero, effectively avoiding the growth of sodium dendrites and improving the safety of the battery.

[0012] Furthermore, the addition amount of tris(trimethylsilyl) phosphate is 0.1 - 1% of the total mass of the sodium salt and the additive, and the addition amount of sodium difluorophosphate is 1 - 5% of the total mass of the sodium salt and the additive.

[0013] Furthermore, the thickness of the sodium titanium phosphate coating on the surface of sodium iron sulfate is 5 - 100 nm.

[0014] Furthermore, the capacity of the active material of the positive electrode is 1.02 - 1.2 times that of the active material of the negative electrode. In the present invention, the capacity of the battery system is controlled by the negative electrode, and the surplus positive electrode capacity ensures the cycle stability. The energy density and power density of the battery are further improved.

[0015] Furthermore, the preparation method of the sodium iron sulfate coated with sodium titanium phosphate includes the following steps:

[0016] S1. Mix the sodium source, titanium source, phosphorus source and citric acid in a non-aqueous solvent to obtain a precursor solution;

[0017] S2. Disperse the sodium iron sulfate powder in the precursor solution, perform ultrasonic treatment, and react at 120 - 150 °C for 6 - 12 h to obtain sodium iron sulfate with in-situ grown sodium titanium phosphate;

[0018] S3. Under a protective atmosphere, anneal at 300 - 400 °C for 1 - 3 h to obtain sodium iron sulfate coated with sodium titanium phosphate.

[0019] Furthermore, in S2, the ultrasonic treatment time is 0.5 - 2 h.

[0020] Furthermore, in S2, after the reaction, it also includes centrifugation, washing, and drying steps under vacuum conditions at 50 - 70 °C.

[0021] Furthermore, in S3, the protective atmosphere is argon.

[0022] Further, in S1, the sodium source, titanium source, and phosphorus source are mixed at an atomic ratio of Na:Ti:P = (0.9 - 1.1):(1.9 - 2.1):(2.9 - 3.1).

[0023] Further, the sodium source is sodium acetate or sodium nitrate, the titanium source is tetrabutyl titanate, and the phosphorus source is triethyl phosphate or ammonium dihydrogen phosphate.

[0024] Further, the non-aqueous solvent is selected from one or more of anhydrous ethanol, ethylene glycol, and N-methylpyrrolidone. The use of a non-aqueous solvent avoids the dissolution of sodium iron sulfate.

[0025] Further, the sodium salt is selected from one or more of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, sodium bis(oxalato)borate, sodium trifluoromethanesulfonate, sodium bis(trifluoromethylsulfonyl)imide, and sodium bis(fluorosulfonyl)imide. The concentration of the sodium salt in the electrolyte is 0.1 - 10 mol / L.

[0026] Further, the organic solvent is selected from one or more of ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, dimethyl carbonate, trimethyl phosphate, triethyl phosphate, and diethylene glycol dimethyl ether.

[0027] Advantages of the present invention:

[0028] In the present invention, the sodium iron sulfate material with a high voltage is used as the positive electrode active material, and sodium titanate phosphate is coated on the surface of sodium iron sulfate to increase its structural stability. At the same time, the coating material sodium titanate phosphate also acts synergistically with the additives in the electrolyte. The phosphate group of the coating material sodium titanate phosphate reacts with tris(trimethylsilyl) phosphate and sodium difluorophosphate to form a solid electrolyte membrane. In this way, the solid electrolyte membrane and the sodium titanate phosphate coating layer provide double protection for the positive electrode, greatly improving the structural stability of the positive electrode.

[0029] In the present invention, the polyanion material sodium titanate phosphate with a highly stable crystal lattice structure is selected as the negative electrode active material. Among them, the potential of the Ti ion to change valence is much higher than that of the Na ion. Therefore, sodium dendrites will not be generated on the negative electrode sodium titanate phosphate, and the sodium ions of the negative electrode active material will not be consumed. At the same time, it will not react with the electrolyte at the interface, and there is no process of SEI film formation-dissolution-reconstruction, and the sodium ions of the electrolyte will not be consumed. In this way, the capacity retention rate of the battery can be effectively improved, and the cycle stability and safety of the battery system can be ensured.

[0030] The battery system of the present invention is stable. There is no process of dendrite formation-dissolution-reconstruction on both the positive electrode and the negative electrode, and the active sodium ions will not be consumed. The current is close to zero during long-term floating charge, effectively avoiding the growth of sodium dendrites and improving the safety of the battery.

[0031] In the battery system of the present invention, the capacity is controlled by the negative electrode, and the surplus positive electrode capacity ensures the cycling stability, further improving the energy density and power density of the battery. Description of the Drawings

[0032] To more clearly illustrate the technical solutions of the present invention, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0033] Figure 1 It is the charge-discharge curve of the sodium-ion battery of Embodiment 1 of the present invention;

[0034] Figure 2 It is the EIS spectrum of sodium titanium phosphate at the negative electrode of Embodiment 1 of the present invention before and after 500 cycles;

[0035] Figure 3 It is the TEM spectrum of sodium titanium phosphate at the negative electrode of Embodiment 1 of the present invention after 500 cycles. Detailed Embodiments

[0036] The technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0037] This embodiment provides a low self-discharge sodium-ion battery, including a positive electrode, a negative electrode, and an electrolyte; the active material of the positive electrode is sodium iron sulfate coated with sodium titanium phosphate, and the active material of the negative electrode is sodium titanium phosphate; the electrolyte includes a sodium salt, an organic solvent, and an additive, and the additive is tris(trimethylsilyl) phosphate and sodium difluorophosphate.

[0038] In this embodiment, a sodium iron sulfate material with a high voltage is used as the positive electrode active material, and sodium titanium phosphate is coated on the surface of sodium iron sulfate to increase its structural stability. At the same time, the coating material sodium titanium phosphate also acts synergistically with the additives in the electrolyte. The phosphate group of the coating material sodium titanium phosphate reacts with tris(trimethylsilyl) phosphate and sodium difluorophosphate to form a solid electrolyte membrane. In this way, the solid electrolyte membrane and the sodium titanium phosphate coating layer provide double protection for the positive electrode, greatly improving the structural stability of the positive electrode. Specifically, TMSP reacts with the phosphate group of sodium titanium phosphate coated on the surface of sodium iron sulfate to generate a silicon-based phosphate solid electrolyte membrane, which can effectively inhibit the dissolution of the positive electrode; NaDFP reacts with the phosphate group of sodium titanium phosphate coated on the surface of sodium iron sulfate to form a fluorine-containing phosphate solid electrolyte membrane, which has certain ionic conductivity and chemical stability and will not reduce the electrochemical performance of the positive electrode material while forming protection, ensuring that the material is suitable for long-term floating charge. The solid electrolyte membrane formed by the reaction of the two additives with sodium titanium phosphate and the sodium titanium phosphate coating layer jointly provide double protection for the positive electrode.

[0039] In this embodiment, the negative electrode active material is a polyanion material sodium titanium phosphate with a highly stable crystal structure. Among them, the potential at which the Ti ion changes its valence is much higher than the Na ion potential. Therefore, no sodium dendrites will be generated in the negative electrode sodium titanium phosphate, and the sodium ions of the negative electrode active material will not be consumed. At the same time, it will not undergo an interfacial reaction with the electrolyte, and there is no process of SEI film formation-dissolution-reconstruction, and the sodium ions of the electrolyte will not be consumed. In this way, the capacity retention rate of the battery can be effectively improved, and the cycle stability and safety of the battery system can be ensured. The battery system is stable, and there is no process of dendrite formation-dissolution-reconstruction in both the positive electrode and the negative electrode, and the active sodium ions will not be consumed. When floating charging for a long time, the current is close to zero, effectively avoiding the growth of sodium dendrites and improving the safety of the battery.

[0040] As a preferred embodiment, the addition amount of tris(trimethylsilyl) phosphate is 0.1-1% of the total mass of the sodium salt and the additives, and the addition amount of sodium difluorophosphate is 1-5% of the total mass of the sodium salt and the additives.

[0041] As a preferred embodiment, the thickness of the sodium titanium phosphate coated on the surface of sodium iron sulfate is 5-100 nm.

[0042] As a preferred embodiment, the active material capacity of the positive electrode is 1.02-1.2 times that of the negative electrode. The capacity of the battery system in this embodiment is controlled by the negative electrode, and the surplus positive electrode capacity ensures the cycle stability. The energy density and power density of the battery are further improved.

[0043] As a preferred embodiment, the preparation method of the sodium iron sulfate coated with sodium titanium phosphate includes the following steps:

[0044] S1. Mix a sodium source, a titanium source, a phosphorus source and citric acid in a non-aqueous solvent to obtain a precursor solution;

[0045] S2. Disperse sodium iron sulfate powder in the precursor solution, perform ultrasonic treatment, and react at 120 - 150 °C for 6 - 12 h to obtain sodium iron sulfate with sodium titanate phosphate grown in-situ;

[0046] S3. Anneal at 300 - 400 °C for 1 - 3 h under a protective atmosphere to obtain sodium iron sulfate coated with sodium titanate phosphate.

[0047] Specifically, in S1, the sodium source, titanium source, and phosphorus source are mixed at an atomic ratio of Na:Ti:P = (0.9 - 1.1):(1.9 - 2.1):(2.9 - 3.1); the sodium source is sodium acetate or sodium nitrate, the titanium source is tetrabutyl titanate, and the phosphorus source is triethyl phosphate or ammonium dihydrogen phosphate; the non-aqueous solvent is selected from one or more of anhydrous ethanol, ethylene glycol, and N-methylpyrrolidone. Selecting a non-aqueous solvent avoids the dissolution of sodium iron sulfate.

[0048] Specifically, in S2, the ultrasonic treatment time is 0.5 - 2 h; after the reaction, it also includes centrifugation, washing, and drying steps under vacuum conditions at 50 - 70 °C. In S3, the protective atmosphere is argon.

[0049] As a preferred embodiment, the sodium salt is selected from one or more of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, sodium bis(oxalato)borate, sodium trifluoromethanesulfonate, sodium bis(trifluoromethylsulfonyl)imide, and sodium bis(fluorosulfonyl)imide, and the concentration of the sodium salt in the electrolyte is 0.1 - 10 mol / L. The organic solvent is selected from one or more of ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, dimethyl carbonate, trimethyl phosphate, triethyl phosphate, and diethylene glycol dimethyl ether.

[0050] Example 1

[0051] This example relates to a low self-discharge sodium-ion battery, including a positive electrode, a negative electrode, and an electrolyte. The preparation method includes the following steps:

[0052] (1) Using tetrabutyl titanate as the titanium source, ammonium dihydrogen phosphate as the phosphorus source, and sodium acetate as the sodium source, dissolve them in anhydrous ethanol at an atomic ratio of Na:Ti:P = 1:2:3, add citric acid to inhibit titanium hydrolysis to obtain a precursor solution; disperse sodium iron sulfate powder in the precursor solution, perform ultrasonic treatment for 30 minutes to ensure uniform dispersion; transfer to a high-pressure reaction kettle and react at 120 °C for 10 hours (solvothermal conditions) to enable in-situ growth of sodium titanate phosphate on the surface of sodium iron sulfate; centrifuge and wash three times with ethanol, and dry under vacuum at 60 °C. Anneal in an inert atmosphere (Ar) at 300 °C for 2 hours to promote the crystallization of the coating layer and avoid the decomposition of sodium iron sulfate, obtaining sodium iron sulfate coated with sodium titanate phosphate.

[0053] (2) Sodium iron phosphate coated with sodium titanium phosphate is used as the positive electrode active material, sodium titanium phosphate is used as the negative electrode active material, the conductive agent is acetylene black, and the binder is a 5 wt.% PVDF NMP solution. The positive and negative electrode sheets are prepared by manual coating according to the mass ratio of active material:conductive agent:binder = 90:5:5.

[0054] (3) Sodium hexafluorophosphate is used as the electrolyte sodium salt with a concentration of 1 mol / L. The solvent is ethylene carbonate and diethyl carbonate with a volume ratio of 1:1, and tris(trimethylsilyl) phosphate and sodium difluorophosphate are added. The addition amounts of the two are 0.5% and 2% of the total mass of the sodium salt and the additive, respectively.

[0055] (4) The positive and negative electrode sheets and the electrolyte are assembled into a soft-pack battery, where the capacity ratio of the positive and negative electrodes is 1.1:1.

[0056] Comparative Example 1

[0057] The difference between this comparative example and Example 1 is that sodium iron sulfate is directly used as the positive electrode active material without coating sodium titanium phosphate, and other parameters and steps remain unchanged to prepare a sodium-ion battery.

[0058] Comparative Example 2

[0059] The difference between this comparative example and Example 1 is that tris(trimethylsilyl) phosphate is not added to the electrolyte, and other parameters and steps remain unchanged to prepare a sodium-ion battery.

[0060] Comparative Example 3

[0061] The difference between this comparative example and Example 1 is that sodium difluorophosphate is not added to the electrolyte, and other parameters and steps remain unchanged to prepare a sodium-ion battery.

[0062] Comparative Example 4

[0063] The difference between this comparative example and Example 1 is that neither tris(trimethylsilyl) phosphate nor sodium difluorophosphate is added to the electrolyte, and other parameters and steps remain unchanged to prepare a sodium-ion battery.

[0064] Comparative Example 5

[0065] The difference between this comparative example and Example 1 is that the capacity ratio of the positive and negative electrodes is 0.9:1, and other parameters and steps remain unchanged to prepare a sodium-ion battery.

[0066] Comparative Example 6

[0067] The difference between this comparative example and Example 1 is that the negative electrode active material is HC (hard carbon), and other parameters and steps remain unchanged to prepare a sodium-ion battery.

[0068] Comparative Example 7

[0069] The difference between this comparative example and Example 1 is that the additive in the electrolyte is replaced by VC (vinyl carbonate), and other parameters and steps remain unchanged to prepare a sodium-ion battery.

[0070] Test Example 1

[0071] (1) The sodium-ion batteries of Example 1 and Comparative Examples 1-7 were placed in an incubator at 45 °C, and the batteries were activated using a Blue Power charge-discharge instrument. The activation voltage range was 0.6 V - 2.2 V, the charge-discharge rate was 0.1 C, and the charge-discharge was carried out for 3 weeks to obtain the charge-discharge curves of the batteries. Among them, the charge-discharge activation curve of Example 1 is as Figure 1 shown. It can be seen that the mid-discharge voltage of the battery is 1.6 V, the charge-discharge curve is normal, and there is no short-circuit phenomenon due to the coating of sodium titanium phosphate on the positive electrode. At the same time, it is also confirmed that the electrolyte additive has no negative effect on the electrochemical performance of the battery.

[0072] (2) To verify the self-discharge rate of the battery, a high-temperature storage experiment was carried out: The battery was fully charged at a rate of 1 C at 45 °C (the charging capacity was CQ1), stored for 28 days, and then discharged. The discharge capacity was DQ1, and the capacity retention rate R1 = DQ1 / CQ1 * 100% was obtained.

[0073] (3) Floating charge performance test:

[0074] a. The capacity of the battery was calibrated at 0.1 C;

[0075] b. After floating charging at 1.75 V for 28 days, the battery was discharged to the cut-off voltage of 0.6 V;

[0076] c. The capacity was calibrated at 0.1 C for 2 cycles;

[0077] d. Repeat steps 2 and 3, and the capacity retention rate R2 of 12 times (one year) of 0.1 C capacity calibration = the discharge capacity of the 12th cycle / the discharge capacity of the 1st cycle * 100% was used to evaluate the floating charge stability of the battery.

[0078] The capacity retention rates R1 and R2 of the sodium-ion batteries of Example 1 and Comparative Examples 1-3 are shown in Table 1.

[0079] Table 1

[0080] Group Capacity retention rate R1 Capacity retention rate R2 Example 1 99.5％ 99.1％ Comparative example 1 92.4％ 85.4％ Comparative example 2 94.6％ 87.9％ Comparative example 3 94.1％ 87.1％ Comparative example 4 93.1％ 86.7％ Comparative example 5 96.7％ 79.8％ Comparative example 6 85.1％ Short circuit occurred for the 6th time Comparative example 7 93.0％ 86.8％

[0081] As can be seen from Table 1, when the battery of Example 1 was stored at 45 °C for 28 days, the energy retention rate R1 was 99.5%, indicating that the self-discharge rate of the battery was only 0.5%. The capacity retention rate R2 was still 98% after one year of floating charge. Calculated according to this attenuation rate, the service life of the battery as a backup power supply can reach 20 years.

[0082] From the comparison between Comparative Example 1 and Example 1, it can be seen that the surface of the positive sodium iron sulfate NFS does not have the coating layer of sodium titanium phosphate NTP. Although there are additives in the electrolyte, since there is no PO4 in the coating layer 3- ions as the "access source", a dense protective film cannot be formed on the surface of the positive electrode. During the floating charge process, iron dissolution will occur in sodium iron sulfate, resulting in a reduction in the amount of active material and capacity attenuation.

[0083] From the comparison between Comparative Examples 2-4 and Example 1, it can be seen that there is no additive in Comparative Example 4, but the NTP protective layer can prevent the positive electrode from being eroded by the electrolyte, so the energy retention rate is slightly higher than that of Comparative Example 1. Only one additive is added in Comparative Examples 2-3, and the capacity retention rates R1 and R2 are both higher than those of Comparative Example 1 and Comparative Example 4. Sodium difluorophosphate is added in Comparative Example 2, and it reacts with phosphate ions to form a solid electrolyte protective film, which is a fluorophosphate. It has both certain chemical stability and ionic conductivity, so R1 and R2 of Comparative Example 2 are slightly higher than those of Comparative Example 3.

[0084] From the comparison between Comparative Example 5 and Example 1, it can be seen that when the capacity of the positive electrode is less than that of the negative electrode and the positive electrode control is adopted, since the floating charge voltage is lower than the charging cut-off voltage, when floating charging, the current decreases, the internal polarization of the battery decreases accordingly, the battery shows an undercharged state, and continuous charging will occur. The OCV of the battery increases accordingly, resulting in the decomposition of the electrolyte and the failure of the battery after long-term cycling.

[0085] From the comparison between Comparative Example 6 and Example 1, it can be seen that when hard carbon is used as the negative electrode active material, during the long-term floating charge process, uneven deposition will occur on the surface of the negative electrode, forming sodium metal dendrites and causing a short circuit.

[0086] In Comparative Example 7, when the commonly used additive VC in lithium batteries is used, the result is similar to that without additives, and there is basically no improvement in the capacity retention rates R1 and R2. This is mainly because the additive VC will not form a protective film with phosphate ions on the surface of the positive electrode during the activation process.

[0087] Figure 2 is the EIS spectrum of sodium titanium phosphate at the negative electrode of Example 1 before and after 500 cycles. It can be seen that the spectrum after 500 cycles (500th) is the same as that before cycling (1st), indicating that no SEI is generated. At the same time, Figure 3 the TEM image of sodium titanium phosphate at the negative electrode after cycling also proves this point.

[0088] The present invention has been described in detail above in combination with specific embodiments and exemplary examples. However, these descriptions should not be construed as limiting the present invention. Those skilled in the art understand that without departing from the spirit and scope of the present invention, various equivalent substitutions, modifications or improvements can be made to the technical solutions and implementation manners of the present invention, and these all fall within the scope of the present invention. The protection scope of the present invention is subject to the appended claims.

Claims

1. A low self-discharge sodium-ion battery, characterized in that, It includes a positive electrode, a negative electrode and an electrolyte; The active material of the positive electrode is sodium iron sulfate coated with sodium titanium phosphate, and the active material of the negative electrode is sodium titanium phosphate; The electrolyte includes a sodium salt, an organic solvent and an additive, and the additive is tris(trimethylsilyl) phosphate and sodium difluorophosphate.

2. The low self-discharge sodium ion battery according to claim 1, characterized in that, The addition amount of tris(trimethylsilyl) phosphate is 0.1-1% of the total mass of the sodium salt and the additive, and the addition amount of sodium difluorophosphate is 1-5% of the total mass of the sodium salt and the additive.

3. The low self-discharge sodium ion battery according to claim 1, wherein The thickness of the sodium titanium phosphate coated on the surface of sodium iron sulfate is 5-100 nm.

4. The low self-discharge sodium ion battery according to claim 1, characterized in that The active material capacity of the positive electrode is 1.02-1.2 times that of the negative electrode.

5. The low self-discharge sodium ion battery according to claim 1, wherein The preparation method of the sodium iron sulfate coated with sodium titanium phosphate includes the following steps: S1. Mix a sodium source, a titanium source, a phosphorus source and citric acid in a non-aqueous solvent to obtain a precursor solution; S2. Disperse the sodium iron sulfate powder in the precursor solution, perform ultrasonic treatment, and react at 120-150 °C for 6-12 h to obtain sodium iron sulfate with in-situ grown sodium titanium phosphate; S3. Under a protective atmosphere, anneal at 300-400 °C for 1-3 h to obtain sodium iron sulfate coated with sodium titanium phosphate.

6. The low self-discharge sodium ion battery according to claim 5, wherein In S1, the sodium source, the titanium source and the phosphorus source are mixed according to an atomic ratio of Na:Ti:P = (0.9-1.1):(1.9-2.1):(2.9-3.1).

7. The low self-discharge sodium ion battery according to claim 5, wherein The sodium source is sodium acetate or sodium nitrate, the titanium source is tetrabutyl titanate, and the phosphorus source is triethyl phosphate or ammonium dihydrogen phosphate.

8. The low self-discharge sodium ion battery according to claim 5, wherein The non-aqueous solvent is selected from one or more of anhydrous ethanol, ethylene glycol, and N-methylpyrrolidone.

9. The low self-discharge sodium ion battery according to claim 1, wherein The sodium salt is selected from one or more of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, sodium bis(oxalato)borate, sodium trifluoromethanesulfonate, sodium bis(trifluoromethylsulfonyl)imide, and sodium bis(fluorosulfonyl)imide, and the concentration of the sodium salt in the electrolyte is 0.1-10 mol / L.

10. The low self-discharge sodium ion battery according to claim 1, characterized in that, The organic solvent is selected from one or more of ethyl methyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, dimethyl carbonate, trimethyl phosphate, triethyl phosphate, and diethylene glycol dimethyl ether.

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