An ester-based electrolyte containing a bromobenzene additive for a sodium metal battery and a sodium metal battery

Abstract

The application provides an ester-based electrolyte containing a bromobenzene additive for a sodium metal battery and a sodium metal battery, the ester-based electrolyte comprising a bromobenzene additive, an ester-based solvent and a sodium salt; the bromobenzene additive accounts for 1-10 vol% of the ester-based electrolyte. The electrolyte realizes spherical growth of sodium and reduces the formation of 'dead sodium'; the amount of the electrolyte additive is small, the effect of improving the sodium metal deposition morphology and cycle stability is remarkable, and the electrolyte has great industrialization potential.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of electrolyte, and particularly relates to an ester-based electrolyte containing bromobenzene additive for a sodium metal battery and a sodium metal battery. BACKGROUND

[0002] Sodium-ion batteries are ideal substitutes for lithium-ion batteries because of their similar chemical properties to lithium, low cost and abundant sodium resources. Among the negative electrode candidate materials for sodium batteries, metallic sodium is considered the most promising anode to achieve high-energy-density sodium batteries due to its high theoretical capacity (1166 mAh g -1 ) and low electrochemical potential (-2.71 V vs. standard hydrogen electrode). However, the main obstacle associated with sodium metal anodes is the safety issue. The formation and growth of sodium dendrites during cycling can lead to short circuits and even the risk of ignition or explosion. In addition, fresh sodium dendrites increase the side reactions between sodium metal and electrolyte, leading to the continuous consumption of both. At the same time, sodium dendrites will produce a large amount of non-active sodium (i.e. dead sodium) due to the loss of electronic contact, ultimately leading to capacity loss and shortened life. So far, various strategies have been developed, such as designing sodium-friendly three-dimensional scaffolds, constructing artificial SEI protective layers to suppress sodium dendrites and improve the performance of sodium metal anodes. To some extent, these strategies can alleviate the dendrite problem by inducing uniform ion flux. However, the time-consuming and complex preparation process hinders their large-scale application. Using additives to optimize electrolyte composition is considered the most cost-effective and feasible strategy.

[0003] So far, some electrolyte additives, such as potassium bis(trifluoromethylsulfonyl)imide (KTFSI), lithium hexafluorophosphate (LiPF6), sodium polysulfide (Na2S6) and cetyltrimethylammonium bromide (CTAB) have been used to stabilize sodium metal anodes in ether-based electrolytes. Compared with ether electrolytes, carbonate electrolytes have low cost and better oxidative stability, making them ideal electrolytes for high-voltage cathodes. However, sodium dendrites are exacerbated in traditional carbonate electrolytes. Therefore, there is a great need to develop effective additives in ester-based electrolytes to suppress sodium dendrites. SUMMARY

[0004] Therefore, the purpose of the present application is to provide an ester-based electrolyte containing bromobenzene additive for a sodium metal battery, which can preferentially reduce SEI layer with high sodium ion diffusion rate on the surface of sodium metal, effectively suppress the growth of sodium dendrites, and thus improve the cycle stability and service life of the battery.

[0005] The present application provides an ester-based electrolyte containing bromobenzene additive for a sodium metal battery, comprising bromobenzene additive, ester-based solvent and sodium salt.

[0006] The bromobenzene additive accounts for 1-10 vol% of the ester-based electrolyte.

[0007] The ester-based electrolyte provided by the application is obtained by adding bromobenzene to a base electrolyte, i.e., using an ester-based solvent and a sodium salt as the base electrolyte.

[0008] In the application, the bromobenzene additive is selected from one or more of monobromobenzene, dibromobenzene, tribromobenzene, tetrabromobenzene, hexabromobenzene and their isomers, and is preferably 1,2-dibromobenzene. In the application, the addition of bromobenzene to the electrolyte effectively inhibits the growth of sodium metal negative electrode dendrites, and sodium grows in a spherical manner. The specific effect is that the bromobenzene additive has stronger oxidizability than the electrolyte solvent, and is preferentially reduced in situ on the surface of the sodium negative electrode to form a SEI layer rich in NaBr. NaBr has a low sodium ion diffusion energy barrier, which accelerates the kinetics of sodium ions passing through the SEI layer, so that there are sufficient Na atoms below the SEI layer, improving the uniformity of sodium deposition and realizing the spherical growth of Na metal, thereby improving the cycle stability of the sodium metal secondary battery.

[0009] The bromobenzene additive accounts for 1-10 vol% of the ester-based electrolyte, preferably 1-5 vol%, and more preferably 3 vol%.

[0010] In the application, the ester-based solvent is selected from one or more of ethylene carbonate, diethyl carbonate, fluorinated ethylene carbonate, dimethyl carbonate, methyl ethyl carbonate, ethylene carbonate, propylene carbonate and ethylene carbonate.

[0011] In the application, the sodium salt is selected from one or more of NaClO4, NaTFSI and NaFSI.

[0012] In the application, the ester-based electrolyte comprises 1,2-dibromobenzene, NaClO4 and an ester-based solvent.

[0013] The ester-based solvent is a mixed solvent of ethylene carbonate, diethyl carbonate and fluorinated ethylene carbonate; in the mixed solvent, the volume ratio of ethylene carbonate to diethyl carbonate is 1:0.95-1.05, and fluorinated ethylene carbonate accounts for 4.5-5.5% of the total mass of the electrolyte. In a specific embodiment, the volume ratio of ethylene carbonate to diethyl carbonate is 1:1, and fluorinated ethylene carbonate accounts for 5% of the total mass of the electrolyte.

[0014] The application provides a sodium metal battery comprising a positive electrode material, a negative electrode material, a separator and an electrolyte.

[0015] The electrolyte is the ester-based electrolyte described in the above technical solution.

[0016] In a specific embodiment of the present invention, NaClO4 is used as the sodium salt in the ester electrolyte, ethylene carbonate, diethyl carbonate, and fluoroethylene carbonate are used as the electrolyte organic solvents, and 1,2-dibromobenzene is used as an additive.

[0017] This invention provides an ester electrolyte containing a bromobenzene additive for sodium metal batteries, comprising a bromobenzene additive, an ester solvent, and a sodium salt; the bromobenzene additive accounts for 1-10 vol% of the ester electrolyte. This electrolyte achieves spherical growth of sodium and reduces the formation of "dead sodium"; the small amount of electrolyte additive significantly improves the morphology and cycle stability of sodium metal deposition, and has great industrialization potential. Attached Figure Description

[0018] Figure 1 The sodium-copper half-cells of Examples 1, 2, 3 and the control group of the present invention are 0.1 mA cm. -2 The current density deposited on the copper foil is 0.2 mAh cm⁻¹. -2 Comparison of SEM morphology of sodium;

[0019] Figure 2 The sodium-sodium symmetric batteries of Examples 1, 2, and 3 of this invention, and the control group, were tested at 1 mA / cm². -2 Current density and 1mAh cm -2 Comparison of cycle stability under capacity test conditions;

[0020] Figure 3 These are transmission electron microscope images of sodium deposition morphology on the copper surface of sodium-copper half-cells in the control group and Example 2 of this invention, wherein (a) is the control group and (b) is Example 2.

[0021] Figure 4 The sodium-sodium symmetric battery used in the control group and Example 2 of this invention operates at 1 mA / cm². -2 Current density and 1mAh cm -2 Comparison of SEM morphology of sodium metal surface after 15 cycles under capacity test conditions, where (a) is the control group and (b) is Example 2;

[0022] Figure 5 This is a comparison chart of the cycle performance of the full cells in Example 2 of the present invention and the control group under the test condition of 10C. Detailed Implementation

[0023] To further illustrate the present invention, the following detailed description, in conjunction with embodiments, provides an ester electrolyte containing bromobenzene additive for sodium metal batteries and a sodium metal battery provided by the present invention, but these descriptions should not be construed as limiting the scope of protection of the present invention.

[0024] The sodium salts, solvents, and electrolyte additive abbreviations used in the following examples are as follows:

[0025] Sodium salt: Sodium perchlorate NaClO4

[0026] Solvents: Ethylene carbonate EC, diethyl carbonate DEC, fluoroethylene carbonate FEC,

[0027] Additives: Bromobenzene, 1,2-dibromobenzene 1,2-DBB.

[0028] The base electrolyte in the examples is 1 mol / L sodium perchlorate dissolved in a mixed solvent of ethylene carbonate, diethyl carbonate, and fluoroethylene carbonate, with the volume ratio of ethylene carbonate to diethyl carbonate being 1:1, and the fluoroethylene carbonate accounting for 5% of the total mass of the electrolyte. The modified electrolyte is the base electrolyte with the addition of an appropriate volume ratio of the additive.

[0029] The CR2032 type button cell is used for evaluation in the examples, and the test conditions for the symmetrical cell are as follows: sodium foil with a diameter of 10 mm is used as the positive and negative electrode, and the test current density and capacity are 1 mA cm -2 and 1 mAh cm -2 , respectively.

[0030] The examples described below each include a corresponding control example. The only difference between the examples and the corresponding control examples is that the electrolyte in the examples contains a bromobenzene additive, while the control examples do not contain an additive.

[0031] Example 1

[0032] In an argon-filled glove box, the base electrolyte is mixed with 1 Vol% of 1,2-DBB in a 10 mL brown glass bottle by magnetic stirring, and the resulting electrolyte is prepared. Then, according to the order of positive cell shell, sodium metal electrode, separator, sodium metal electrode (or copper foil), negative cell shell, the prepared electrolyte is injected, and a sodium-sodium symmetrical cell (or a sodium-copper half-cell) is assembled in the glove box. The sodium-sodium symmetrical cell is tested for cycle stability using a current density of 1 mA cm -2 and a capacity of 1 mAh cm -2 . The sodium-copper half-cell is used to deposit 0.2 mAh cm -2 of sodium on the copper foil using a current density of 0.1 mA cm -2 , to observe the deposition morphology. As shown in b of Figure 1 , the sodium deposition morphology is smoother in the experimental group using the ester electrolyte containing 1% 1,2-DBB additive, but there are still a small amount of dendrites. In addition, as shown in Figure 2As shown, the sodium-sodium symmetric cell can be cycled stably for more than 184 hours, while the control group failed after only 155 h. This indicates that the additive has a significant effect in ester-based electrolytes.

[0033] Example 2

[0034] The difference between this example and Example 1 is only that the amount of 1,2-DBB is adjusted to 3%, and other conditions and parameters are exactly the same as in the example. In addition, according to the order of positive cell shell, copper mesh (or copper foil), separator, sodium metal electrode, negative cell shell, the electrolyte to be used is injected, and a half-cell is assembled in a glove box for in-situ preparation of TEM samples. Further, the present application also assembled a full cell with sodium vanadium phosphate as the positive electrode. As shown in Figure 1 As shown in FIG. 1C, sodium forms a smooth deposition layer on the copper surface, and the deposition layer is relatively dense. Such smooth deposition morphology greatly improves the cycle performance of the sodium-sodium symmetric cell. As shown in Figure 2 As shown, using the electrolyte containing 3% 1,2-DBB additive, the sodium-sodium symmetric cell can be cycled stably for more than 600 h. As shown in Figure 3 As shown, using the electrolyte designed by us, the sodium deposition morphology is spherical, thereby avoiding the growth of dendrites. In order to further verify the mechanism of spherical growth, the sodium-sodium symmetric cell is cycled at a current density of 1 mA cm -2 and a capacity of 1 mAh cm -2 After 15 cycles, the surface morphology of the cycled sodium metal negative electrode is observed by SEM. As shown in Figure 4 As shown, using the electrolyte containing 3% 1,2-DBB additive, the sodium metal surface can achieve uniform stripping, and after further magnification, spherical small particles can be observed on the surface. However, for the control group, due to the uneven stripping of the sodium metal surface, holes and the like appear, and further local magnification can see that there are a large number of dendrites on the surface. As shown in Figure 5 As shown, thanks to the spherical deposition morphology, the full cell using the designed electrolyte can be cycled stably for 2000 cycles under the test condition of 10C, while the control group fails only after 652 cycles.

[0035] Example 3

[0036] The difference between this example and Example 1 is only that the amount of 1,2-DBB is adjusted to 5%, and other conditions and parameters are exactly the same as in the example. The sodium-sodium symmetric cell is cycled at a current density of 1 mA cm -2 and a capacity of 1 mAh cm -2 The sodium-copper half-cell is deposited on the copper foil at a current density of 0.1 mA cm -2 and a capacity of 0.2 mAh cm -2 The deposition morphology is observed. As shown in Figure 1As shown in FIG. 5, in the experimental group using the ester electrolyte containing 5% 1,2-DBB additive, although the sodium deposition morphology is smooth, there are some cracks in the micro-morphology. In addition, as shown in FIG. 6, the sodium-sodium symmetric battery can be stably cycled for more than 490 hours. Compared with Example 2, when the additive amount is 5%, the cycle life decreases slightly and the polarization also increases. Therefore, it is indicated that the appropriate proportion of the additive in the ester electrolyte is 3%. Figure 2

[0037] From the above examples, it can be seen that in the glove box filled with argon and with water and oxygen values less than 0.1 ppm, different volume ratios of bromobenzene additive are added to the base electrolyte to prepare electrolytes with additive volume percentages of 1%, 3%, and 5% for the controlled deposition of sodium metal. The present application forms a SEI film rich in NaBr on the surface of the sodium metal negative electrode by preferentially reducing the bromobenzene additive, accelerates the transport rate of sodium ions in the SEI film, ensures the sufficiency of the sodium ion source below the SEI film layer, promotes the uniform deposition of sodium ions on the negative electrode surface, realizes the spherical growth of sodium, inhibits the growth of sodium dendrites, and greatly improves the cycle life of the sodium-sodium symmetric battery and the full battery matched with the sodium vanadium phosphate.

[0038] The above description is only the preferred embodiments of the present application, and it should be pointed out that for ordinary skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, and these improvements and refinements should also be considered as the protection scope of the present application.​

Claims

1.A bromobenzene additive-containing ester electrolyte for a sodium metal battery, wherein the ester electrolyte comprises 1, 2-dibromobenzene, NaClO 4 and an ester-based solvent; the ester-based solvent is a mixed solvent of ethylene carbonate, diethyl carbonate and fluoroethylene carbonate; and the bromobenzene additive accounts for 1-5 vol% of the ester electrolyte. 2.A sodium metal battery, comprising a positive electrode material, a negative electrode material, a separator and an electrolyte; and the electrolyte is the ester electrolyte according to claim 1. ​ ​ ​ ​

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