Innovative Selenium-Enhanced Sulfur Frameworks for High-Performance Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are excellent energy storage systems with high theoretical energy density, low cost, and environmental compatibility. However, they suffer from the limitation of the “shuttle effect,” which arises from the dissolution and migration of lithium polysulfides (LiPS) intermediates (Li2Sn, 4 ≤ n ≤ 8) within the electrolyte during the charge and discharge cycles. This phenomenon leads to rapid capacity fading, poor Coulombic efficiency, and ultimately a limited cycle life of the battery. The shuttle effect is believed to be caused by the poor electronic and ionic conductivity of sulfur and the inherent instability of the LiPS. Previously methods to mitigate the shuttle effect such as introducing host materials that can provide physical confinement and chemical absorption of sulfur cathodes, however, these strategies have not completely resolved the issue because they do not prevent the initial formation of long-chain LiPS. As a result, researchers are investigating others materials, such as organosulfur compounds, which incorporate short-chain sulfur atoms to limit the formation of LiPS. However, these materials also suffer from low sulfur content, complex synthesis procedures, and insufficient capacity for practical applications. To this end, new study published in Advanced Materials and conducted by Dr. Lu Chen, Dr. Ting He, PhD candidate Kexuan Liao, graduate student Hang Lu, PhD candidate Jian Ma, Dr. Yutong Feng, PhD candidate Shuo Meng, Professor Chi Zhang, and Professor Jinhu Yang from the school of Chemical Science and Engineering at Tongji University developed a novel class of sulfur-containing ternary covalent inorganic frameworks (CIFs) and created a cathode material that provide high capacity, excellent retention and also effectively suppresses the shuttle effect at its source.

The team synthesized the new CIF, P₄Se₆S₄₀, through a simple comelting method of phosphorus (P), sulfur (S), and selenium (Se) powders and was designed to leverage the unique properties of Se which enhance the electronic conductivity of sulfur chains and can act as a barrier to prevent the formation of long-chain polysulfides. Additionally, the stable formation of Li₃PS₄ during lithiation served as both an ionic conductor and a chemical anchor, which also inhibit the shuttle effect. The authors used scanning electron microscopy and transmission electron microscopy and showed the P₄Se₆S₄₀ particles had an average size of approximately 5 μm and an amorphous structure. Energy-dispersive X-ray spectroscopy confirmed the uniform distribution of P, Se, and S within the framework. Further analysis using X-ray diffraction patterns showed no obvious peaks for P₄Se₆S₄₀, indicating its amorphous nature. They also assessed the electrochemical performance of the P₄Se₆S₄₀ cathode and compared it with P₄S₄₀, Se₆S₄₀, and pure sulfur cathodes. The first discharge-charge curves showed a significant difference with P₄Se₆S₄₀ exhibited a main cathodic peak at 2.03 V, which indicates the formation of short-chain Li₂S and Li₂Se, whereas P₄S₄₀ displayed two cathodic peaks associated with the production of high-order polysulfides. The cyclic voltammetry profiles highlighted the smaller voltage difference between cathodic and anodic peaks for P₄Se₆S₄₀ (0.26 V) compared to P₄S₄₀ (0.35 V), which suggested reduced polarization. Electrochemical impedance spectroscopy measurements further confirmed the enhanced conductivity of P₄Se₆S₄₀ due to Se’s presence, resulting in lower internal resistance. Moreover, the authors evaluated the cycling performance of P₄Se₆S₄₀ at various current densities and found that at 0.1 A g⁻¹, the P₄Se₆S₄₀ cathode delivered an initial discharge capacity of 652 mAh g⁻¹ and retained 535 mAh g⁻¹ after 600 cycles, with a high capacity retention of 82%. In comparison, the P₄S₄₀ cathode showed significant capacity fading, retaining only 284 mAh g⁻¹ after 600 cycles. The researchers tested also the practical applicability and its performance of P₄Se₆S₄₀ at high mass loadings and observed at an areal loading of 10.5 mg cm−2, P₄Se₆S₄₀ achieved a gravimetric capacity of 594 mAh g⁻¹ and an areal capacity of 6.3 mAh cm⁻², significantly exceeding the benchmark for commercial lithium-ion batteries. After 50 cycles, the cathode retained 525 mAh g⁻¹ and 5.5 mAh cm⁻², corresponding to a cycle retention rate of 87.6%.

In conclusion, the study conducted by Professors Chi Zhang and Jinhu Yang and their team at Tongji University addressed some of the most persistent challenges in the field, such as the shuttle effect and poor electronic/ionic conductivity of sulfur cathodes. Their innovative material design is simple in synthesis and scalable which are major advantages. Moreover, the high specific capacity of the P₄Se₆S₄₀ cathode is expected to produce a higher energy density in Li-S batteries which can be critical in portable electronics and electric vehicles. Furthermore, the successful suppression of the shuttle effect and the improvement cycling stability achieved in the study mean that Li-S batteries with P₄Se₆S₄₀ cathodes can have a significantly longer operational lifespan which will result in reducing the frequency of battery replacements, which is a significant advantage for both consumers and industries relying on battery-powered devices and systems.