A sodium ion battery electrolyte for high voltage layered oxide cathodes

Abstract

The present invention belongs to the technical field related to sodium ion batteries, and discloses a sodium ion battery electrolyte for a high-voltage layered oxide positive electrode, wherein the electrolyte comprises an organic solvent, an electrolyte salt and an additive, wherein the organic solvent is a carbonate-based solvent that does not contain ethylene carbonate; the electrolyte salt is a sodium salt; the additive comprises a positive electrode film-forming additive and a negative electrode film-forming additive, wherein the positive electrode film-forming additive comprises at least one of sodium tetraphenylborate, sodium tetrafluoroborate, sodium difluorooxalatoborate, and trimethylsilyl phosphate, and the negative electrode film-forming additive comprises at least one of fluoroethylene carbonate, vinyl sulfate, and tetravinylsilane. The electrolyte can effectively reduce transition metal dissolution and electrolyte decomposition, improve charge and discharge efficiency, and thus improve the cycle stability of layered oxide sodium ion batteries at higher voltages.

Description

Technical Field

[0001] The present invention belongs to the technical field related to sodium ion batteries, and more specifically, relates to a sodium ion battery electrolyte for a high-voltage layered oxide positive electrode. Background Art

[0002] With the gradual scarcity of traditional fossil energy and increasingly serious environmental problems, the development of new renewable energy has become an inevitable trend. At present, electric vehicles and smart grid energy storage systems are developing rapidly. Secondary batteries, as a high-efficiency energy storage technology, have the characteristics of fast response speed and high energy density. However, limited lithium resources have become the main bottleneck restricting the large-scale energy storage application of lithium-ion batteries. The development of resource-rich and environmentally friendly sodium-ion batteries is of great strategic significance.

[0003] The working principle of sodium ion batteries is the same as that of lithium ion batteries. During the charge and discharge process, sodium ions are embedded and extracted between the positive and negative electrodes of the battery. Sodium ions have a larger ion radius than lithium ions, so they require larger ion channels. Traditional lithium storage electrode materials may not be suitable for the storage of sodium ions. In recent years, domestic and foreign scientific and technological personnel have carried out a lot of research work on the development of new sodium storage materials and have made good progress. Layered oxide sodium positive electrode materials such as Na1 / 3Ni1 / 3Mn2 / 3O2 have high capacity and have received great attention in recent years as extremely potential sodium storage materials. However, their practical application is limited by problems such as high charging voltage window, electrolyte decomposition caused by transition metal dissolution, low charge and discharge efficiency, and poor cycle stability. Summary of the invention

[0004] In view of the above defects or improvement needs of the prior art, the present invention provides a sodium ion battery electrolyte for a high-voltage layered oxide positive electrode, which can effectively reduce transition metal dissolution and electrolyte decomposition, improve the charge and discharge efficiency, and thereby improve the cycle stability of the layered oxide sodium ion battery at a higher voltage.

[0005] To achieve the above-mentioned purpose, according to one aspect of the present invention, there is provided a sodium ion battery electrolyte for a high-voltage layered oxide positive electrode, the electrolyte comprising an organic solvent, an electrolyte salt and an additive, wherein the organic solvent is a carbonate-based solvent that does not contain ethylene carbonate; the electrolyte salt is a sodium salt; the additive comprises a positive electrode film-forming additive and a negative electrode film-forming additive, wherein the positive electrode film-forming additive comprises at least one of sodium tetraphenylborate (NaBPh4), sodium tetrafluoroborate (NaBF4), sodium difluorooxalatoborate (NaDFOB), and trimethylsilyl phosphate (TMSP), and the negative electrode film-forming additive comprises at least one of fluoroethylene carbonate (FEC), vinyl sulfate (DTD), and tetravinylsilane (TVS).

[0006] Preferably, the organic solvent is at least two of propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0007] Preferably, the concentration of the electrolyte salt is 0.3-2.0 mol / L.

[0008] Preferably, the positive electrode film-forming additive accounts for 0.5% to 5% of the total mass of the total volume of the electrolyte, and the negative electrode film-forming additive accounts for 0.5% to 10% of the total mass of the electrolyte.

[0009] Preferably, the electrolyte salt is sodium hexafluorophosphate or sodium bis(fluorosulfonyl)imide.

[0010] Preferably, the electrolyte salt is sodium hexafluorophosphate.

[0011] Preferably, the positive electrode film-forming additive is trimethylsilyl phosphate or sodium tetrafluoroborate.

[0012] Preferably, the negative electrode film-forming additive is fluoroethylene carbonate or vinyl sulfate.

[0013] Preferably, the content of the positive electrode film-forming additive is 1 wt %, and the content of the negative electrode film-forming additive is 2 wt %.

[0014] In general, compared with the prior art, the above technical solution conceived by the present invention has the following beneficial effects in the sodium ion battery electrolyte for high voltage layered oxide positive electrode provided by the present invention:

[0015] 1. Since sodium ions have a larger ionic radius than common lithium ions, sodium is released from the layered oxide, resulting in an unstable positive electrode structure, and then the dissolution of transition metals far exceeds that of lithium-ion batteries. The dissolution of transition metals easily catalyzes the decomposition of EC and deposits on the sodium negative electrode to destroy the negative electrode interface film, resulting in a decrease in the battery's coulombic efficiency. The use of carbonate-based solvents without ethylene carbonate (EC) as organic solvents can reduce the interfacial side reactions involving EC, thereby improving the overall charge and discharge efficiency and cycle life of the battery; at the same time, the positive electrode film-forming additives selected have a higher HOMO, and the negative electrode film-forming additives have a lower LUMO, which can preferentially decompose at the positive and negative electrode-electrolyte interfaces during the first charge and discharge process to form an interface film. Stabilizing the electrode-electrolyte interface can reduce the dissolution of transition metals and further improve the cycle life of layered oxide sodium-ion batteries.

[0016] 2. The concentration of the electrolyte salt described in this application is 0.3~2.0M to meet the sodium ion conductivity during long cycles; the positive electrode film-forming additive accounts for 0.5%~5% of the total volume and mass of the electrolyte, and the negative electrode film-forming additive accounts for 0.5%~10% of the total mass of the electrolyte. Below this range, the film-forming effect is not obvious, and above this range, the film is too thick or there is residual additive, all of which are not conducive to long-term cycle stability.

[0017] 3. The electrolyte salt is sodium hexafluorophosphate or sodium bis(fluorosulfonyl)imide, and more preferably the electrolyte salt is sodium hexafluorophosphate, because it has good dynamic performance, safety performance and low cost, and will not corrode aluminum foil.

[0018] 4. The cathode film-forming additive is preferably trimethylsilyl phosphate (TMSP) or sodium tetrafluoroborate. In addition to forming a film, trimethylsilyl phosphate can also remove H2O / HF in the electrolyte through the Si-O functional group, reduce the corrosion of HF on the cathode material, and further reduce the dissolution of transition metals, further improving the cycle life of layered oxide sodium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is the first charge and discharge curve of the Na1 / 3Ni1 / 3Mn2 / 3O2 / Na battery assembled with the electrolyte of the present invention (HV-LO-E: NaPF6-EMC / DMC-NaBF4-FEC) and the comparative sample (Commercial-E: NaPF6-EC / DMC);

[0020] Figure 2 It is the cycle performance diagram of Na1 / 3Ni1 / 3Mn2 / 3O2 / Na battery assembled with the electrolyte of the present invention (HV-LO-E: NaPF6-EMC / DMC-NaBF4-FEC) and the comparative sample (Commercial-E: NaPF6-EC / DMC);

[0021] Figure 3 It is the coulombic efficiency diagram of the Na1 / 3Ni1 / 3Mn2 / 3O2 / Na battery assembled with the electrolyte of the present invention (HV-LO-E: NaPF6-EMC / DMC-NaBF4-FEC) and the comparison sample (Commercial-E: NaPF6-EC / DMC). DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and 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. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0023] The present invention provides a sodium ion battery electrolyte for a high-voltage layered oxide positive electrode, the electrolyte comprising an organic solvent, an electrolyte salt and an additive. The high voltage is a voltage greater than 4V.

[0024] The organic solvent is a carbonate-based solvent that does not contain ethylene carbonate (EC). In a further preferred embodiment, the organic solvent is at least two of propylene carbonate, diethyl carbonate, ethyl methyl carbonate, and dimethyl carbonate, that is, a combination of a high dielectric constant solvent and a low viscosity solvent is required.

[0025] The electrolyte salt is a sodium salt, which may be sodium hexafluorophosphate (NaPF6) or sodium bis(fluorosulfonyl)imide (NaFSI), and is more preferably sodium hexafluorophosphate (NaPF6), which ensures safety performance and low cost without corroding aluminum foil.

[0026] The concentration of the electrolyte salt is 0.3-2.0 mol / L.

[0027] The additives include positive electrode film-forming additives and negative electrode film-forming additives, wherein the positive electrode film-forming additives include at least one of sodium tetraphenylborate (NaBPh4), sodium tetrafluoroborate (NaBF4), sodium difluorooxalatoborate (NaDFOB), and trimethylsilyl phosphate (TMSP), and the negative electrode film-forming additives include at least one of fluoroethylene carbonate (FEC), vinyl sulfate (DTD), and tetravinylsilane. Such a combination has a lower LUMO and a higher HOMO, and can preferentially decompose at the positive and negative electrode-electrolyte interface during the first charge and discharge process to form an interface film, stabilize the electrode-electrolyte interface, and TMSP has the function of removing H2O / HF, which can reduce the dissolution of transition metals and further improve the cycle life of layered oxide sodium ion batteries.

[0028] The positive electrode film-forming additive accounts for 0.5% to 5% of the total mass of the total volume of the electrolyte, and the negative electrode film-forming additive accounts for 0.5% to 10% of the total mass of the electrolyte.

[0029] The electrolyte solution described above can be obtained by adding electrolyte salt and additives to an anhydrous organic solvent and stirring evenly. The anhydrous organic solvent is prepared by adding an organic solvent to a dehydrating agent and standing for 2-4 days. The dehydrating agent is a molecular sieve with a model of and Any one of the types.

[0030] The Na1 / 3Ni1 / 3Mn2 / 3O2 / Na battery assembled with the electrolyte (HV-LO-E: NaPF6-EMC / DMC-NaBF4-FEC) of the present invention has a first coulombic efficiency of up to 99% and an average coulombic efficiency of up to 99.9% at a voltage of 2.5-4.13V, and a capacity retention rate of 99% after 100 cycles at a 1C rate; while the first coulombic efficiency of the comparative sample (Commercial-E: NaPF6-EC / DMC) is only 81%, the average coulombic efficiency is 90%, and the specific capacity during the cycle is lower than that of HV-LO-E. In addition, the carbonate system solvent and electrolyte salt involved in the present invention have low costs and broad market prospects.

[0031] Example

[0032] See Table 1 for details

[0033]

[0034]

[0035]

[0036] Table 1

[0037] from Figure 1 It can be seen that the electrolyte of the present invention (corresponding to Example 4) has a higher first charge and discharge efficiency, that is, the first charge and discharge reversibility is better.

[0038] from Figure 2 By comparison with the embodiments of the invention, it can be seen that the average coulombic efficiency of the embodiments of the invention excluding EC is greater than 99%, while the average coulombic efficiency of the comparison sample Commercial-E is only 90%, which is very unfavorable for the battery cycle life and capacity, and also illustrates the deterioration effect of EC on the performance of layered oxide sodium-ion batteries.

[0039] It can be seen from Examples 1-5 that the best effect is achieved when the sodium salt concentration is 1M. It can be seen from the comparison between Example 4 and Example 6 that the effect is better when the sodium salt is sodium hexafluorophosphate, because sodium hexafluorophosphate has better corrosion resistance to aluminum foil. It can be seen from the comparison between Example 4 and Examples 7-8 that the effect is better when the solvent ratio EMC:DMC=70:30. It can be seen from the comparison between Example 4 and Examples 9-15 that the effect is better when the solvent system is a binary mixture EMC:DMC=70:30. It can be seen from the comparison between Example 4 and Examples 16-24 that the effect is better when the additive contains both positive electrode film-forming additives and negative electrode film-forming additives. It can be seen from the comparison between Examples 1-3 that only one solvent is contained, and the average coulombic efficiency and discharge specific capacity are lower than those of the embodiments of the present invention.

[0040] At 2.5~4.13V charge and discharge voltage, Figure 3 As shown, the average coulombic efficiency of the Na1 / 3Ni1 / 3Mn2 / 3O2 / Na battery assembled with the sodium ion battery electrolyte (HV-LO-E) suitable for high voltage layered oxide positive electrode of the present invention is significantly higher than that of the commercial electrolyte (Commercial-E), and the discharge specific capacity after 100 cycles is also higher; Table 1 is a comparison table of the average coulombic efficiency and discharge specific capacity of the battery with the HV-LO-E electrolyte prepared by the present invention and the Commercial-E electrolyte after 100 cycles at 2.5-4.13V. It can be seen that when EMC:DMC=70:30 (volume ratio), the sodium salt is 1M NaPF6, and the additive is 2% FEC+1% NaBF4, the average coulombic efficiency is as high as 99.9%, and the discharge specific capacity is 86mAh / g.

[0041] In summary, the carbonate ester without ethylene carbonate (EC) described in the present invention is used as a solvent, at least one of sodium hexafluorophosphate and sodium bis(fluorosulfonyl)imide is used as a sodium salt, at least one of sodium tetraphenylborate, sodium tetrafluoroborate, sodium difluorooxalatoborate, and trimethylsilyl phosphate is used as a positive electrode film-forming additive, and at least one of fluoroethylene carbonate, vinyl sulfate, and tetravinylsilane is used as a negative electrode film-forming additive. The prepared electrolyte is suitable for high-voltage layered oxide sodium ion batteries, and has higher first effect, higher average coulomb efficiency and discharge specific capacity.

[0042] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A sodium ion battery electrolyte for a high voltage layered oxide positive electrode, characterized in that: The electrolyte includes an organic solvent, an electrolyte salt and an additive, wherein the organic solvent is a carbonate-based solvent that does not contain ethylene carbonate; the electrolyte salt is a sodium salt; the additive includes a positive electrode film-forming additive and a negative electrode film-forming additive, wherein the additive is at least one of a combination of sodium tetrafluoroborate and fluoroethylene carbonate (NaBF4+FEC), a combination of sodium difluorooxalatoborate and tetravinylsilane (NaDFOB+TVS), and a combination of sodium tetraphenylborate and vinyl sulfate (NaBPh4+DTD).

2. The electrolyte according to claim 1, characterized in that The organic solvent is at least two of propylene carbonate, diethyl carbonate, ethyl methyl carbonate and dimethyl carbonate.

3. The electrolyte according to claim 1, characterized in that The concentration of the electrolyte salt is 0.3-2.0 mol / L.

4. The electrolyte according to claim 1 or 3, characterized in that The positive electrode film-forming additive accounts for 0.5% to 5% of the total mass of the electrolyte, and the negative electrode film-forming additive accounts for 0.5% to 10% of the total mass of the electrolyte.

5. The electrolyte according to claim 1, characterized in that The electrolyte salt is sodium hexafluorophosphate or sodium bis(fluorosulfonyl)imide.

6. The electrolyte according to claim 5, characterized in that The electrolyte salt is sodium hexafluorophosphate.

7. The electrolyte according to claim 1, characterized in that The negative electrode film-forming additive is fluoroethylene carbonate or vinyl sulfate.

8. The electrolyte according to claim 4, characterized in that The content of the positive electrode film-forming additive is 1 wt %, and the content of the negative electrode film-forming additive is 2 wt %.

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