Li-S Batteries with Li₂S Cathodes and Si/C Anodes

Abstract

Lithium-sulfur (Li-S) batteries promise improved capacities over lithium ion batteries. While currently mostly metallic lithium anodes are used, the use of silicon-anodes might offer better safety and durability. However, in a lithium-sulfur-Silicon (Li-S-Si) battery, lithium must be introduced either on the anode or on the cathode in form of Li 2 S. In this study, we have prepared Li 2 S cathodes in combination with Si anodes (i.e., Si/Li 2 S full-cells) to investigate both the processes during initial charging/activation of Li 2 S cathodes and the effect of Li 2 S cathode activation on the cycling performance of Si/Li 2 S full-cells. We observed that the initial activation requires a substantially higher charging potential than for the subsequent cycles. In situ XRD analysis of the cathode during the first cycle clearly indicates the gradual transformation of Li 2 S to polysulfides and finally to crystalline sulfur, i.e., even large Li 2 S particles (≈20 μm) can be charged completely. The result is further confirmed by ex-situ SEM/EDS analysis, which revealed the formation of large sheets of sulfur at the cathode/separator interface. Similar cycling performance of Si/Li 2 S full-cells is observed at both 0.1 C and 1 C rates, a clear advantage over Li/Li 2 S cells, which suffer from severe dendrite formation at 1 C in the case of high Li 2 S loadings. High capacity energy storage systems are needed for various applications ranging from portable electronic devices to automotive applications. For the latter, a safe onboard energy storage system that can provide sufficient driving range is needed. However, the specific energy of current intercalation based lithium ion batteries (≈250-280 Wh/kg cell ) substantially limits the driving range compared to that of conventional fuel vehicles 1,2 so that batteries offering higher specific energy are required.Lithium-air (Li-O 2 ) 3 and lithium-sulfur (Li-S) 4,5 batteries are among the most widely explored so-called post-lithium ion technologies, for which the lithium ions in the cathode react with either oxygen or sulfur during discharge, resulting in Li 2 O 2 or Li 2 S discharge product, respectively. Considering the high theoretical specific energy of these post-lithium ion cathodes on the active materials level (viz., ≈3.5 kWh/kg Li2O2 for Li-O 2 and ≈2.5 kWh/kg Li2S for Li-S 1 ), they are very promising for use in electric vehicles, even though the specific energy gains over lithium ion batteries if compared on the battery system level are substantially lower. 6 However, there are still several major issues to be resolved. In the case of Li-O 2 batteries, these include the poor charge/discharge reversibility 7,8 as well as the poor stability of electrolytes 9-11 and of the catalyst support. 12 Similarly, Li-S batteries are still plagued by an irreversible loss of active sulfur species, by polysulfide shuttling, and by the continuous electrolyte consumption at the lithium electrode.13,14 Nevertheless, Li-S battery performance/durability has improved significantly over ...

Understanding the Charging Mechanism of Lithium-Sulfur Batteries Using Spatially Resolved Operando X-Ray Absorption Spectroscopy

“…The inset demonstrates that formation of S 8 can be detected as early as after 10% of the charging process, and that the concentration of S 8 is increasing throughout the entire process. Our sensitivity to detecting S 8 in both the separator and the cathode is a significant improvement over previous operando studies based on XRD characterization, which could detect formation of S 8 only after 60% of the charging process, 8 or not at all. 9 Furthermore, our results demonstrate that during the first charge occurring at a significant overpotential, polysulfides do not exist in a detectable concentration, and that it is possible to extract the entire theoretical capacity by converting Li 2 S to S 8 .…”

“…In this proposed mechanism, the electrochemically generated S 8,solid subsequently dissolves and diffuses toward a Li 2 S particle. Then, S 8,solution and Li 2 S react chemically to produce S n 2− in trace concentrations, and S n 2− converts through a series of chain-growth/disproportionation reactions to a polysulfide species that can be electrochemically oxidized to solid S 8 (∼1% of the theoretical capacity). At the same time, other parts of the micro-meter sized Li 2 S particles could become oxidized through a series of chemical reactions (Equations 1-2), leading to production of polysulfides that can be electrochemically oxidized to S 8 (Equation 3), thus, enabling the extraction of the entire theoretical capacity.…”

“…7,8 Specifically, it has been recently reported that a Li-S battery, which is assembled in a discharged state using a Li 2 S cathode, requires an application of a high overpotential in the very first charge, even though each subsequent charge can be performed at a lower potential.…”

“…z E-mail: yelena.gorlin@tum.de only been characterized using X-ray diffraction (XRD), 8,9 a technique that is capable of detecting crystalline solids, but cannot provide information about amorphous species. X-ray absorption spectroscopy (XAS) is an alternative operando characterization technique that is especially suited for the characterization of Li-S batteries, because it is capable of detecting solid S 8 and Li 2 S as well as polysulfides dissolved in the electrolyte.…”

“…X-ray absorption spectroscopy (XAS) is an alternative operando characterization technique that is especially suited for the characterization of Li-S batteries, because it is capable of detecting solid S 8 and Li 2 S as well as polysulfides dissolved in the electrolyte. 21,22 To date, operando XAS has only been applied to the study of Li-S batteries assembled in a charged state using S 8 cathodes, [21][22][23][24][25] and has not been used to identify intermediates during the initial charge of Li 2 S. Since the initial charging process differs significantly from all the subsequent charges, XAS characterization of both the starting species and the intermediates associated with these charging processes has an opportunity to facilitate a significantly improved mechanistic understanding of the operation of Li-S batteries.…”