Lithium-sulfur (Li-S) batteries hold their promises as a high energy-density and low-cost energy storage system. The commercialization of this technology, however, has been hindered by several issues such as polysulfides shuttle effect, sluggish redox reaction kinetics, and Li dendrite growth.

This work begins with modeling the charging process of Li-S batteries to describe and investigate the dual voltage platform phenomenon. The redox mediation and size effect coupled with electrochemical reactions is exploited for describing the Li₂S activation process. It is demonstrated that promoting the dissolution of Li₂S particles, including enhancing the redox mediation rate and controlling Li₂S deposition size, can greatly enhance the redox reaction kinetics. To inhibit the polysulfi...[ Read more ]

Lithium-sulfur (Li-S) batteries hold their promises as a high energy-density and low-cost energy storage system. The commercialization of this technology, however, has been hindered by several issues such as polysulfides shuttle effect, sluggish redox reaction kinetics, and Li dendrite growth.

This work begins with modeling the charging process of Li-S batteries to describe and investigate the dual voltage platform phenomenon. The redox mediation and size effect coupled with electrochemical reactions is exploited for describing the Li₂S activation process. It is demonstrated that promoting the dissolution of Li₂S particles, including enhancing the redox mediation rate and controlling Li₂S deposition size, can greatly enhance the redox reaction kinetics. To inhibit the polysulfides shuttling, a carbonized polypyrrole coated Fe₃O₄ yolk-shelled sulfur host is designed and fabricated. The battery with this host, possessing much-alleviated polysulfide shuttling by virtue of the physical barrier from carbon shell and strong interaction between polysulfides and Fe₃O₄ core, demonstrates a long lifespan of 1000 cycles at 0.5C with a S-loading of 1.5 mg cm^-2.

To further enhance the battery performance, an Egg-waffle like MXene-based nanostructure is developed as a sulfur host. In this nanostructure, the macropores facilitate ion transport, and the polysulfides are immobilized via the strong chemisorption from titanium carbide. The battery with this host achieves a capacity of 260 mAh g^-1 even at an ultrahigh current rate of 10C (13.4 mA cm^-2), and a stable cyclability over 1000 cycles at 1C is delivered. However, the sulfur loading is still limited by the sluggish reaction kinetics. Thus, a hierarchical hollow Co-CNT@MXene nanostructure is developed for high S-loading Li-S batteries. In this architecture, the cobalt, supported by the conductive carbon nanotube, accelerates polysulfides conversion kinetics, and the nitrogen dopants enhance the chemisorption of sulfur species on MXene. The achieved multiplexed functionality enables a long lifespan of 170 cycles with a capacity retention of 85.8% at 0.2C with an ultrahigh S-loading of 6 mg cm^-2.

The Li-S full battery requires a reversible and safe Li-based anode. We first look at the Li electrode structure design. By taking advantage of the unique application of the femtosecond laser technique on the MXene membrane, the lithiophilic TiO₂ nanoparticles are spatial-selectively synthesized in the aligned microchannels. Benefiting from the built-in fluorine terminals in MXene, a LiF-reinforced SEI is in-situ formed upon plating/stripping. Meanwhile, the exposed nanometer-scale edges inside the channels can manipulate the electric field, thus achieving a favorable Li⁺ ion migration into the channels as verified by the finite element simulation (FEM). Consequently, the lithium deposition is well-controlled, and a stable performance of 500 cycles with Coulombic efficiency of 95.9 % is achieved at 20 mA cm^-2 and 1 mAh cm^-2. This well-designed Li electrode also enables a long cycle life (300 cycles) at 1C when paired with the carbon/sulfur cathode. To further construct a bifunctional artificial protective layer for simultaneously suppressing the parasitic reactions and inhibiting the dendritic formation, a two-dimensional material graphitic carbon nitride (g-C₃N₄) is coated on the lithium metal surface. According to the DFT calculation and spectroscopic studies, g-C₃N₄ has a strong binding with solvated Li⁺ ion, thus homogenizing the Li⁺ ion distribution and eliminating the lithium dendrite growth. Moreover, the insulating layer avoids the direct contact between highly reactive lithium metal and dissolved polysulfides as a physical shield, thereby inhibiting the side reactions. The coating layer significantly enhances the cyclability to 200 cycles of the battery at 0.1C.

Keywords: Li-S battery; polypyrrole; MXene; femtosecond laser technique; graphitic carbon nitride; anode protection; lithium deposition; energy density; power density