Making the Unfeasible Feasible: Synthesis of the Battery Material Lithium Sulfide via the Metathetic Reaction between Lithium Sulfate and Sodium Sulfide

Abstract

Lithium sulfide (Li 2 S) is a highly desired material for advanced batteries. However, its current industrial production is not suitable for large-scale applications in the long run because the process is carbon-emissive, energy-intensive, and cost-ineffective. This article demonstrates a new method that can overcome these challenges by reacting lithium sulfate (Li 2 SO 4 ) with sodium sulfide. This approach, which seems unfeasible initially because Li 2 SO 4 is barely soluble in ethanol at room temperature, becomes feasible when heated ethanol and an excess amount of Li 2 SO 4 are used. More interestingly, product purification is easier than that in other metathetic reactions, thanks to the poor solubility of Li 2 SO 4 . In order to further minimize the overall costs of producing Li 2 S, the concomitant byproduct LiNaSO 4 and the unfinished precursor Li 2 SO 4 are converted into more valuable materials, Li 2 CO 3 and Na 2 SO 4 . Moreover, the homemade Li 2 S is competitive with the commercial Li 2 S in cathode performance and gains further enhancement when being composited with the Co 9 S 8 catalyst. Thus, this Li 2 SO 4 -based metathesis of Li 2 S has great potential for practical applications.

Preparation of high-quality lithium sulfide by reducing lithium sulfate with hydrogen: a green and cost-effective method

“…To date, the methods used to synthesize high-quality lithium sulfide include the lithium–sulfur method, 13–19 the complex decomposition reaction method, 20–25 the magnesium thermal reduction method 26 and the carbon thermal reduction method. 27–29 Among them, commercially available high-quality Li 2 S is usually prepared using the lithium–sulfur method, the basic principle of which is to dissolve lithium metal in benzene organic solvent, and then pass H 2 S gas to obtain high-quality Li 2 S. Duchardt et al 13 added lithium metal and S to a tetrahydrofuran (THF) solution of naphthalene, and then reduced the naphthalene to a free-radical anion using the lithium metal, and then used the free-radical anion to reduce S to obtain Li 2 S. Meanwhile, Fan Wu et al 14 carried out a similar operation to prepare Li 2 S. They dissolved lithium metal in the dimethyl ether (Energy Chemical, 99%) solvent of the aromatic hydrocarbon biphenyl (Alfa, 99%) as a lithium source, then added sulfur monomers to obtain crude Li 2 S, and finally washed with dimethyl ether and dried to obtain pure Li 2 S. Zhao and others 15–18 used lithium metal to react with ethanol to obtain lithium–ethanol, and then passed H 2 S through it to obtain Li 2 S. However, the above methods involved use of expensive lithium metal and organic solvents, which further increased the production cost of Li 2 S. Therefore, it is particularly important to explore the low-cost synthesis strategy of high-quality Li 2 S. Subsequently, there has been a series of explorations using low-cost inorganic lithium salts instead of organolithium salts.…”

“…27–29 Among them, commercially available high-quality Li 2 S is usually prepared using the lithium–sulfur method, the basic principle of which is to dissolve lithium metal in benzene organic solvent, and then pass H 2 S gas to obtain high-quality Li 2 S. Duchardt et al 13 added lithium metal and S to a tetrahydrofuran (THF) solution of naphthalene, and then reduced the naphthalene to a free-radical anion using the lithium metal, and then used the free-radical anion to reduce S to obtain Li 2 S. Meanwhile, Fan Wu et al 14 carried out a similar operation to prepare Li 2 S. They dissolved lithium metal in the dimethyl ether (Energy Chemical, 99%) solvent of the aromatic hydrocarbon biphenyl (Alfa, 99%) as a lithium source, then added sulfur monomers to obtain crude Li 2 S, and finally washed with dimethyl ether and dried to obtain pure Li 2 S. Zhao and others 15–18 used lithium metal to react with ethanol to obtain lithium–ethanol, and then passed H 2 S through it to obtain Li 2 S. However, the above methods involved use of expensive lithium metal and organic solvents, which further increased the production cost of Li 2 S. Therefore, it is particularly important to explore the low-cost synthesis strategy of high-quality Li 2 S. Subsequently, there has been a series of explorations using low-cost inorganic lithium salts instead of organolithium salts. Yang et al 20 dissolved LiCl and Na 2 S in anhydrous ethanol at room temperature to generate Li 2 S. Besides, Na 2 S and other lithium salts (LiNO 3 , LiBr, LiI 21 and Li 2 SO 4 22 ) can also be used to prepare Li 2 S. However, a big disadvantage of this metathesis reaction is that it will generate another heterogeneous material, which will lead to complex purification processes. For example, in order to improve the conversion rate of Na 2 S, excess LiCl needs to be added, and the LiCl that does not participate in the reaction is difficult to remove from the Li 2 S, thus affecting the purity and electrochemical properties of the product.…”