Qiushi Miao

Sulfurized polyacrylonitrile (SPAN) has been studied as an alternative cathode to elemental sulfur (S) for Li-S batteries due to its superior cycling stability. Increasing the sulfur content is crucial in raising its specific capacity. However, the factors that determine the charging and discharging rate performance of SPAN, particularly as a function of sulfur content, remain unclear. In this work, we study two SPAN materials with different sulfur content: SPAN and a toluene extracted SPAN. We also evaluate them in carbonated-based and concentrated ether-based electrolytes. We show that unlike oxide cathodes, the diffusion kinetics in SPAN materials is largely independent of state of charge (SoC) within most of the SoC range, which contributes to its superior rate performance. Furthermore, we discover that the presence of extractable sulfur improves charge transfer kinetics but reduces diffusion rate. Finally, electrolyte compositions impact charge transfer but not diffusion rates. This is consistent with the more resistive cathode electrolyte interface layer formed in carbonates but diffusion in SPAN is indeed a solid state process that is not dependent on electrolytes. Our work provides insights for SPAN materials design to obtain high charging and discharging rate performance.

The formation of uniform, nondendritic seeds is essential to realizing dense lithium (Li) metal anodes and long-life batteries. Here, we discover that faceted Li seeds with a hexagonal shape can be uniformly grown on carbon-polymer composite films. Our investigation reveals the critical role of carbon defects in serving as the nucleation sites for their formation. Tuning the density and spatial distribution of defects enables the optimization of conditions for faceted seed growth. Raman spectral results confirm that lithium nucleation indeed starts at the defect sites. The uniformly distributed crystalline seeds facilitate low-porosity Li deposition, effectively reducing Li pulverization during cycling and unlocking the fast-charging ability of Li metal batteries. At a 1 C rate, full cells using LiNi_0.8Mn_0.1Co_0.1O₂ cathode (4.5 mA h cm^-2) paired with a lithium anode grown on carbon composite films achieve a 313% improvement in cycle life compared to baseline cells. Polymer composites with carbonaceous materials rich in defects are scalable, low-cost substrates for high-rate, high-energy-density batteries.

Sulfurized polyacrylonitrile (SPAN) is a promising cathode material for lithium-sulfur (Li-S) batteries due to its significantly reduced polysulfide (PS) dissolution compared to that of elemental S cathodes. Although conventional carbonate-based electrolytes are stable with SPAN electrodes, they are unstable with Li metal anodes. Recently, localized high-concentration electrolytes (LHCEs) have been developed to improve the stability of Li anodes. Here, we report a new strategy to further improve the performance of Li||SPAN batteries by replacing the conventional solvating solvent 1,2-dimethoxyethane (DME) in LHCEs with a new solvating solvent, 1,2-diethoxyethane (DEE). The new optimal DEE-LHCE exhibits less reactivity against Li₂S₂, alleviates PS dissolution, forms a better cathode-electrolyte interphase layer on the SPAN cathode, and enhances SPAN structural reversibility even at elevated temperatures (45 °C). Compared to DME-LHCE, DEE-LHCE with the same salt and diluent leads to better performance in Li||SPAN batteries (with 82.9% capacity retention after 300 cycles at 45 °C), preservation of the SPAN cathode structure, and suppression of volume change of the Li metal anode. A similar strategy on tailoring the solvating solvents in LHCEs can also be used in other rechargeable batteries to improve their electrochemical performances.