Toward Practical Solid-State Lithium–Sulfur Batteries: Challenges and Perspectives

Abstract

Conspectus The energy density of the ubiquitous lithium-ion batteries is rapidly approaching its theoretical limit. To go beyond, a promising strategy is the replacement of conventional intercalation-type materials with conversion-type materials possessing substantially higher capacities. Among the conversion-type cathode materials, sulfur constitutes a cost-effective and earth-abundant element with a high theoretical capacity that has a potential to be game-changing, especially within an emerging solid-state battery configuration. Employment of nonflammable solid electrolytes that improves battery safety and boosts the energy density, as lithium metal anodes are also viable. The long-standing inherent problem of conventional lithium–sulfur batteries, arising from the reaction intermediates dissolved in liquid electrolytes, can be eliminated with inorganic solid ion conductors. In particular, the highly conducting and easily processable lithium-thiophosphates have successfully enabled the lab-scale solid-state lithium–sulfur cells to achieve close-to-theoretical capacities. For applications requiring safe, energy-dense, lightweight batteries, solid-state lithium–sulfur batteries are an ideal choice that could surpass conventional lithium-ion batteries. Nevertheless, there are challenges specific to practical solid-state lithium–sulfur batteries, beyond the typical challenges inherent to solid-state batteries in general. While the conversion reaction of sulfur realizes a large specific capacity, the associated significant total volume changes of the active material results in contact losses among the cathode components and, consequently, decreases reversible capacity. Additionally, the ionically and electronically insulating active material requires composite formation with solid electrolytes and electron-conductive additives to secure sufficient ion and electron supply at a triple-phase boundary. However, the compositing process itself makes the carrier transport pathways very tortuous and requires the balancing of carrier transport and optimization of the attainable energy density. Lastly, the requirement of a high interfacial area to establish sufficient triple-phase boundaries promotes the degradation of the solid electrolytes, and the formation of less-conductive interphases further deteriorates the transport in the composites. This Account focuses on the challenges associated with developing practical solid-state lithium–sulfur batteries and provides an overview over recently developed concepts to tackle these critical challenges: (1) Introduction of the conversion efficiency to enable quantitative assessments of the impact of chemo-mechanical failure. (2) For long-term cycling, the electrolyte degradation at the interface and the electrochemical activity of the formed interphases come into play. Practical stability tests with increased interfacial areas and subsequently altered reversal potentials can quantify the magnitude of the electrolyte degradation and confirm influences of reversible redox activity o...

Will lithium‐sulfur batteries be the next beyond‐lithium ion batteries and even much better?

“…By far, solid‐state electrolytes (SSEs) have undoubtedly gained substantial momentum and are regarded as the “once for all” strategy to solve both the polysulfide shuttle and dendritic growth issues (Figure 3C). SSEs are also among the best solutions to settle the battery safety issues, where for example the use of organic liquid electrolytes in LSBs is the main course 46 . Together with a set of well‐designed electrodes, all solid‐state lithium‐sulfur batteries (ASSLSBs) would be the future “holy grail” for EVs and other transport systems in large‐scale.…”

“…(A,B) Summary of the academic achievement of ASSLSBs and current industrial achievement of quasi‐solid LSBs, 95 as well as the predicted energy densities after step by step modification toward thin ASSLSBs, summarized from References 46, 96.…”

“…SSEs are also among the best solutions to settle the battery safety issues, where for example the use of organic liquid electrolytes in LSBs is the main course. 46 Together with a set of well-designed electrodes, all solid-state lithium-sulfur batteries (ASSLSBs) would be the future "holy grail" for EVs and other transport systems in large-scale. Nonetheless, for high-energy-density and low-cost LSBs, the near-future development would be mainly with high sulfur loading/ lean electrolyte for the liquid types.…”

“…In the roadmap to the practical ASSLSBs, a visualized scenario is to improve the energy density by the step-bystep approach, which is predicated on the basis of the discussion presented above and related data available in literature 46,96 as shown in Figure 7B, where the energy densities are about 300 Wh kg À1 . For further improvement, efforts shall be focused on thinning the electrolyte and raising the sulfur loading.…”

Strategies towards High Performance Lithium‐Sulfur Batteries

“…The electrolyte chemistry and the ability to stabilize LiPSs species play an indispensable role in improving the performances of LSBs. There are four major types of electrolytes commonly used in LSBs such as organic liquid electrolytes, [86] polymer electrolytes, [87] inorganic solid‐state electrolytes, [88] and organic/inorganic composite electrolytes, [89] as shown in Figure 6(a).…”