Sulfide Nanomaterials for Lithium- and Sodium-Ion Battery Electrodes

Abstract

The escalating climate crisis demands an urgent shift from fossil fuels to renewable energy sources such as wind and solar power, to reduce carbon emissions and mitigate global warming. However, the intermittent nature of these renewable sources presents significant challenges to energy stability and reliability. In this context, advanced battery technologies, particularly lithium-ion batteries, have become indispensable. These batteries facilitate the storage of energy, thereby balancing supply and demand, and are pivotal in driving the transition from internal combustion engines to electric vehicles. However, the development of batteries with greater energy density, faster charging times and longer lifespans is critical to bolster this transition and improve the market competitiveness of electric vehicles. The discovery and optimisation of new electrode materials is key to enabling next-generation energy storage solutions. While traditional graphite anodes offer stability, they are hampered by relatively low energy density. In contrast, materials such as silicon, phosphorous and sulfur offer significantly higher theoretical capacities—nearly tenfold that of graphite in the case of silicon. However, the deployment of these materials in commercial batteries faces substantial hurdles, chiefly their considerable volume expansion and contraction during charge and discharge cycles. This can lead to rapid electrode degradation and premature battery failure. To combat these issues, innovative approaches have emerged such as reducing the size of the active materials to the nanoscale. This enhances both the capacity and rate performance of the electrode, due to increased specific surface area and improved ion transport. Furthermore, combining these active materials with conductive nanomaterials, such as carbon nanotubes, yields composite electrodes with significantly improved mechanical strength, and enhanced durability and lifespan. In this work, four high-theoretical capacity but little-studied materials are examined: As2S3, As4S4, AsSbS3, and Sn2S3. Liquid-phase exfoliation techniques were employed to produce dispersions of nanomaterials by ultrasonication of the bulk crystals in isopropanol, marking the first time that this technique had been used for each of these materials. The optical properties of the nanomaterial dispersions were analysed using UV-Visible spectroscopy. The crystallinity and morphology of the materials before and after exfoliation were examined using X-ray diffraction, and Raman spectroscopy was used to explore any structural alterations induced by the exfoliation process. Scanning and transmission electron microscopy, along with atomic force microscopy, provided detailed insights into the morphology of the materials and facilitated the compilation of statistical information on the nanomaterial size distributions. To evaluate their performance in battery electrodes, the exfoliated nanomaterials were incorporated into nanocomposite films containing 20% carbon nanotubes and tested in lithium-ion and sodium-ion half-cells. Cyclic voltammetry was used to analyse the nature and reversibility of the electrochemical reactions during charge and discharge cycles, and galvanostatic charge–discharge experiments were performed to assess the capacity and stability of the electrodes over multiple cycles. The rate performance of the half-cells were evaluated by measuring the capacities obtained across a range of charge and discharge currents and fitting to a semi-empirical rate equation developed by our research group. Theoretical or near-theoretical capacities were achieved in all lithium-ion cells, and in sodium-ion cells of AsSbS3 and Sn2S3. The formation of Na2S was found to be irreversible in the As2S3 and As4S4 sodium-ion cells, but capacities corresponding to the full utilisation of their arsenic component were achieved.