Joule

Joule

Volume 5, Issue 9, 15 September 2021, Pages 2323-2364
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Review
Electrolyte solutions design for lithium-sulfur batteries

https://doi.org/10.1016/j.joule.2021.06.009Get rights and content
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Context & scale

Lithium-sulfur (Li-S) batteries, a promising next-generation energy storage system, has yet to realize the expected cycling life and energy density. The effect of electrolyte solutions on sulfur electrochemistry is monumental, probably more so than in any other system. Realizing long-lived and high-energy Li-S batteries requires a careful redesign of the electrolyte solution.

Polysulfide solubility is one of the most important metrics for Li-S electrolyte solutions. This review evaluates the electrolyte solution chemistry and analyzes the polysulfide solvation behavior therein. The solutions are classified fundamentally into moderately, sparingly, and highly solvating electrolyte solutions (MSEs, SSEs, and HSEs, respectively) on the basis of their solvation power. These electrolyte solutions bear positive and negative effects on the performance, in a variety of ways. MSEs (the conventional dilute ethers) confer arguably the best rate capability but cause extensive polysulfide shuttle and lithium corrosion; SSEs suppress the shuttle and stabilize lithium but might induce relatively more sluggish reaction kinetics; and HSEs promote three-dimensional Li2S deposition but have serious side reactions with lithium. Manipulating the polysulfide solubility provides an important strategy to effectively decrease the electrolyte solution quantity for high cell-level energy density. We pay particular attention to discuss the idea that the solution chemistry, polysulfide solubility, sulfur redox kinetics, electrolyte quantity, and lithium stability are delicately interwoven in Li-S electrochemistry.

To push Li-S batteries forward to their full potential, high-sulfur-loading cathodes must be used in electrolyte evaluation. Screening solvents and salts assisted by machine learning and data science probably helps to accelerate the process and understanding the underlying sulfur reaction pathway by operando techniques can further promote the development of new electrolyte solutions for Li-S batteries.

Summary

Lithium-sulfur (Li-S) batteries promise high energy density for next-generation energy storage systems, yet many challenges remain. Li-S batteries follow a conversion chemistry, which radically differs from intercalation-based lithium-ion batteries. Recently, it has become clear that the chemistry of electrolyte solutions and their ability to stabilize polysulfide Li2Sx species formed by sulfur reduction have a critical effect on energy density and cycling performance. This review evaluates the key role of solution properties and polysulfide solvation. Factors that determine the solvation are discussed, including the solvent, salt, concentration, and interaction with Li-polysulfide species. Three fundamental types of electrolyte solution—moderately (conventional), sparingly, and highly solvating—are presented along with a multi-dimensional analysis of solution chemistry, polysulfide solubility, sulfur reaction pathway, Li2S deposition, and solution quantity. The stability of lithium metal anodes with these solutions is discussed with respect to side reactions, protective surface film formation, and dendritic Li deposition. Emphasis is placed on options to reduce the electrolyte solution/sulfur ratio and prolong battery cycle life. The advantages and disadvantages of the three systems are compared in accordance with the multifaceted requirements. In conclusion, we offer our perspective for future development of Li-S batteries.

Keywords

lithium-sulfur batteries
electrolyte solutions
polysulfide solvation
lithium anode stability
lean electrolyte systems

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