First principles study of layered scandium disulfide for use as Li-ion and beyond-Li-ion batteries

The growing demand for high efficiency portable batteries has prompted a deeper exploration for alternative cathode materials. Due to low Earth abundance, scandium has not received much attention, however its low atomic mass makes it ideal for high gravimetric capacity electrodes. Here we have performed a comprehensive first-principles study to assess the performance of layered ScS₂ as a potential cathode for lithium-ion and beyond-lithium-ion batteries. We have explored the configuration space of ScS₂ and its intercalated compounds using a mix of machine learning and ab initio techniques, finding the ground state geometry to be layered in nature. This layered structure is found to have a high voltage, reaching above 4.5 V for Group I intercalants, ideal volume expansions below 10% for lithium and magnesium intercalation, is electronically conductive, and is ductile once intercalated. Of the intercalants considered, we find that lithium is the best choice for cathode applications, for which we have used a combination of thermodynamic phase diagrams, ab intio phonon calculations, and evaluation of the elastic tensor to conclude that ScS₂ possesses a reversible capacity of 182.99 mA h g^-1, on par with current state of the art cathode materials such as LiCoO₂, NMC, and NCA. Finally, we substitute foreign metal species into the ScS₂ material to determine their effect on key cathode properties, but find that these are overall detrimental to the performance of ScS₂. This does, however, highlight the potential for improvement if scandium were mixed into other layered systems such as the layered transition metal oxides.

Anomalous Discharge Behavior of Graphite Nanosheet Electrodes in Lithium-Oxygen Batteries

Lithium-oxygen (Li-O₂) batteries require rational air electrode concepts to achieve high energy densities. We report a simple but effective electrode design based on graphite nanosheets (GNS) as active material to facilitate the discharge reaction. In contrast to other carbon forms we tested, GNS show a distinctive two-step discharge behavior. Fundamental aspects of the battery’s discharge profile were examined in different depths of discharge using scanning electron microscopy and electrochemical impedance spectroscopy. We attribute the second stage of discharge to the electrochemically induced expansion of graphite, which allows an increase in the discharge product uptake. Raman spectroscopy and powder X-ray diffraction confirmed the main discharge product to be Li₂O₂, which was found as particulate coating on GNS at the electrode top, and in damaged areas at the bottom together with Li₂CO₃ and Li₂O. Large discharge capacity comes at a price: the chemical and structural integrity of the cathode suffers from graphite expansion and unwanted byproducts. In addition to the known instability of the electrode–electrolyte interface, new challenges emerge from high depths of discharge. The mechanistic origin of the observed effects, as well as air electrode design strategies to deal with them, are discussed in this study.

Revealing the Reaction Mechanism of Sodium Selenide Confined within a Single-Walled Carbon Nanotube: Implications for Na-Se Batteries

The sodium-selenium (Na-Se) battery is a competitive candidate as the practical next-generation energy storage device. A Na₁₆Se₈ cluster confined within a (10, 10) single-walled carbon nanotube is constructed to reveal the nanoconfinement effect on the reaction mechanism of the Na-Se battery cathode. It is found that the nanoconfinement can enhance the electronic conductivity of Na _x≥12Se₈ nanostructures because itinerant electrons appear under this condition. During desodiation, polyselenide chains grow longer and the intermediate products become insulators for transferring electrons. However, hole polarons have the potential to act as charge carriers in Na _x≤10Se₈ nanostructures. The open-circuit voltage profile is plotted, and the voltage window is 1.67 ≤ U ≤ 1 V. After the first charge cycle, the cathode cannot discharge to Na₁₆Se₈, but the reversible specific capacity can still arrive at 302 mA h/g of the cathode composite.