Paving the way for using Li₂S batteries

Lithium Sulfide as Cathode Materials for Lithium-Ion Batteries: Advances and Challenges

Due to the ever-growing demand for high-density energy storage devices, lithium-ion batteries with a high-capacity cathode and anode are thought to be the next-generation batteries for their high energy density. Lithium sulfide (Li2S) is considered the promising cathode material for its high theoretical capacity, high melting point, affordable volume expansion, and lithium composition. This review summarizes the activation and lithium storage mechanism of Li2S cathodes. The design strategies in improving the electrochemical performance are highlighted. The application of the Li2S cathode in full cells of lithium-ion batteries is discussed. The challenges and new directions in commercial applications of Li2S cathodes are also pointed out.

Anionic Redox Chemistry in Polysulfide Electrode Materials for Rechargeable Batteries

Classical Li-ion battery technology is based on the insertion of lithium ions into cathode materials involving metal (cationic) redox reactions. However, this vision is now being reconsidered, as many new-generation electrode materials with enhanced reversible capacities operate through combined cationic and anionic (non-metal) reversible redox processes or even exclusively through anionic redox transformations. Anionic participation in the redox reactions is observed in materials with more pronounced covalency, which is less typical for oxides, but quite common for phosphides or chalcogenides. In this Concept, we would like to draw the reader's attention to this new idea, especially, as it applies to transition-metal polychalcogenides, such as FeS₂ , VS₄ , TiS₃ , NbS₃ , TiS₄ , MoS₃ , etc., in which the key role is played by the (S-S)^2- /2 S^2- redox reaction. The exploration and better understanding of the anion-driven chemistry is important for designing advanced materials for battery and other energy-related applications.

Insight on the Li2S electrochemical process in a composite configuration electrode

A novel, low cost and environmentally sustainable lithium sulfide-carbon composite cathode, suitably prepared by combining polyethylene oxide (PEO), LiCF₃SO₃ and Li₂S-C powders is here presented. The cathode is characterized in lithium-metal cell employing a solution of LiCF₃SO₃ salt in dioxolane-dimethylether (DOL-DME) as the electrolyte. Detailed NMR investigation of the diffusion properties of the electrolyte is reported in order to determine its suitability for the proposed cell. The addition of LiNO₃ to the electrolyte solution allows practical application in a lithium sulfur cell using the Li₂S-C-based cathode characterized by a specific capacity of about 500 mAh g^-1 (as referenced to the Li₂S mass). The cell holds its optimal performances for over 70 cycles at C/5 rate, with a steady state efficiency approaching 99%. X-ray diffraction patterns of the cell upon operation suggest the reversibility of the Li₂S electrochemical process, while repeated electrochemical impedance spectroscopy (EIS) measurements indicate the suitability of the electrode-electrolyte interface in terms of low and stable cell impedance. Furthermore, the EIS study clarifies the activation process occurring at the Li₂S cathode during the first charge process, leading to the decrease of the cell polarization during the following cycles. The data here reported shed light on important aspects to be considered for the efficient application of the Li₂S cathode in lithium battery.

PVP-Assisted Synthesis of Uniform Carbon Coated Li2S/CB for High-Performance Lithium-Sulfur Batteries

The lithium-sulfur (Li-S) battery is a great alternative to the state-of-the-art lithium ion batteries due to its high energy density. However, low utilization of active materials, the insulating nature of sulfur or lithium sulfide (Li2S), and polysulfide dissolution in organic liquid electrolyte lead to low initial capacity and fast performance degradation. Herein, we propose a facile and viable approach to address these issues. This new approach entails synthesis of Li2S/carbon black (Li2S/CB) cores encapsulated by a nitrogen-doped carbon shell with polyvinylpyrrolidone (PVP) assistance. Combining energy-filtered transmission electron microscopy (EFTEM) elemental mappings, XPS and FTIR measurements, it is confirmed that the as-synthesized material has a structure of a Li2S/CB core with a nitrogen-doped carbon shell (denoted as Li2S/CB@NC). The Li2S/CB@NC cathode yields an exceptionally high initial capacity of 1020 mAh/g based on Li2S mass at 0.1 C with stable Coulombic efficiency of 99.7% over 200 cycles. Also, cycling performance shows the capacity decay per cycle as small as 0.17%. Most importantly, to further understand the materials for battery applications, field emission transmission electron microscopy (FETEM) and elemental mapping tests without exposure to air for Li2S samples in cycled cells are reported. Along with the first ever FETEM and field emission scanning electron microscopy (FESEM) investigations of cycled batteries, Li2S/CB@NC cathode demonstrates the capability of robust core-shell nanostructures for different rates and improved capacity retention, revealing Li2S/CB@NC designed here as an outstanding system for high-performance lithium-sulfur batteries.

Li2S Nanocrystals Confined in Free-Standing Carbon Paper for High Performance Lithium-Sulfur Batteries

Lithium sulfide (Li2S) with a high theoretical capacity of 1166 mAh g(-1) is a promising cathode material for Li-S batteries as it allows for the use of lithium-free anodes. However, a large overpotential (~1 V) is usually needed to activate microsized Li2S particles due to their low electronic and ionic conductivities. Here, nano-Li2S/carbon paper electrodes are developed via a simple Li2S solution filtration method. Li2S nanocrystals with a size less than 10 nm are formed uniformly in the pores of carbon paper network. These electrodes show an unprecedented low potential difference (0.1 V) in the first and following charges, also show high discharge capacities, good rate capability, and excellent cycling performance. More specifically, the nano-Li2S/carbon nanotube paper electrodes show a reversible capacity of 634 mAh g(-1) with a capacity retention of 92.4% at 1C rate from the 4th to 100th cycle, corresponding to a low capacity fading rate of 0.078% per cycle. These results demonstrate a facile and scalable electrode fabrication process for making high performance nano-Li2S/carbon paper electrodes, and the superior performance makes them promising for use with lithium metal-free anodes in rechargeable Li-S batteries for practical applications.

Insight into sulfur reactions in Li-S batteries

Understanding and controlling the sulfur reduction species (Li2Sx, 1 ≤ x ≤ 8) under realistic battery conditions are essential for the development of advanced practical Li-S cells that can reach their full theoretical capacity. However, it has been a great challenge to probe the sulfur reduction intermediates and products because of the lack of methods. This work employed various ex situ and in situ methods to study the mechanism of the Li-S redox reactions and the properties of Li2Sx and Li2S. Synchrotron high-energy X-ray diffraction analysis used to characterize dry powder deposits from lithium polysulfide solution suggests that the new crystallite phase may be lithium polysulfides. The formation of Li2S crystallites with a polyhedral structure was observed in cells with both the conventional (LiTFSI) electrolyte and polysulfide-based electrolyte. In addition, an in situ transmission electron microscopy experiment observed that the lithium diffusion to sulfur during discharge preferentially occurred at the sulfur surface and formed a solid Li2S crust. This may be the reason for the capacity fade in Li-S cells (as also suggested by EIS experiment in Supporting Information ). The results can be a guide for future studies and control of the sulfur species and meanwhile a baseline for approaching the theoretical capacity of the Li-S battery.