Sicen Yu

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.

In this study, we investigate the rate performance limits in LiNi_0.6Mn_0.2Co_0.2O₂ cathodes for lithium-ion batteries, focusing on how cathode thickness, porosity, and threshold voltage impact discharge capacity. By conducting galvanostatic discharge experiments across a wide range of current densities using cathodes of varying thicknesses and porosities, we identify two distinct rate-limiting mechanisms: ionic liquid-diffusion and Ohmic/charge-transfer limitations. Our findings show that thick, dense cathodes are primarily limited by lithium diffusion in the electrolyte, while thinner and more porous cathodes are dominated by Ohmic/charge-transfer limitations, particularly at higher lower cutoff voltages. Crucially, these results allow us to identify the rate-limiting mechanisms in different cathode configurations, offering clear insights into how cathode design can be optimized for improved performance. Understanding these mechanisms is essential for designing next-generation batteries with enhanced rate performance, which is critical for applications such as electric vehicles and renewable energy storage systems.

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.

Redox flow batteries (RFBs) are promising choices for stationary electric energy storage. Nevertheless, commercialization is impeded by high-cost electrolyte and membrane materials. Here, we report a low-cost all-iron RFB that features inexpensive FeSO4 electrolytes, microporous membrane along with a glass fiber separator. The addition of 0.1 м 1-ethyl-3-methylimidazolium chloride (EMIC) overcomes the low solubility of FeSO4 in water which is raised to 2.2 м. DFT calculations demonstrate that EMI+ can strengthen the interaction between sulfate anions and water molecules. This electrolyte composition also allows both anode and cathode reactions to operate without actively maintaining a pH gradient between them, thus eliminating the need for expensive ion exchange membranes. The all-iron RFB demonstrates stable operation at a current density of 20 mA cm−2 for more than 800 cycles via a simple, periodic regeneration process. Furthermore, the system cost of FeSO4/EMIC RFBs is projected to be $ 50 per kWh due to its low-cost active materials and the inexpensive microporous membrane. This low-cost, high-concentration all-iron RFB is a promising stationary energy‐storage system for storing renewable energy.

We present a feasible method for the preparation of one-dimensional N-doping carbon nanofibers encapsulated NaTi₂(PO₄)₃ (NTP-NCNFs) through electrospinning accompanied by calcination. The poor electrical conductivity of NTP is significantly improved and the as-prepared NTP-NCNFs exhibit stable and ultrafast sodium-storage capability. The NTP-NCNFs maintains a stable specific capacity of 121 mAh g⁻¹ at 10 C after 2,000 cycles, which only drop to 105 mAh g⁻¹ after 20,000 cycles. Furthermore, the NTP-NCNFs show excellent rate performance from 0.2 to 20 C, whose recovery efficiency still reaches 99.43%. The superior electrochemical property is mainly attributed to the large specific surface area, high porosity, N-doping carbon coating, and one-dimensional structure of NTP-NCNFs.