A review of size engineering-enabled electrocatalysts for Li-S chemistry

Li-S batteries (LSBs) have received extensive attention owing to their remarkable theoretical capacity (1672 mA h g^-1) and high energy density (2600 W h kg^-1), which are far beyond those of the state-of-the-art Li-ion batteries (LIBs). However, the retarded sulfur reaction kinetics and fatal shuttle effect have hindered the practical implementations of LSBs. In response, constructing electrocatalysts for Li-S systems has been considered an effective strategy to date. Particularly, size engineering-enabled electrocatalysts show high activity in the sulfur redox reaction, considerably contributing to the latest advances in Li-S system research. In this tutorial review, we provide a systematic summary of nano- to atomic-scale electrocatalysts employed in Li-S chemistry, aiming at figuring out the working mechanism of size engineering-enabled electrocatalysts in the sulfur redox reaction and guiding the rational construction of advanced LSBs toward practically viable applications.

Semi-Flooded Sulfur Cathode with Ultralean Absorbed Electrolyte in Li-S Battery

Lean electrolyte (small E/S ratio) is urgently needed to achieve high practical energy densities in Li-S batteries, but there is a distinction between the cathode's absorbed electrolyte (AE) which is cathode-intrinsic and total added electrolyte (E) which depends on cell geometry. While total pore volume in sulfur cathodes affects AE/S and performance, it is shown here that pore morphology, size, connectivity, and fill factor all matter. Compared to conventional thermally dried sulfur cathodes that usually render "open lakes" and closed pores, a freeze-dried and compressed (FDS-C) sulfur cathode is developed with a canal-capillary pore structure, which exhibits high mean performance and greatly reduces cell-to-cell variation, even at high sulfur loading (14.2 mg cm-2) and ultralean electrolyte condition (AE/S = 1.2 µL mg-1). Interestingly, as AE/S is swept from 2 to 1.2 µL mg-1, the electrode pores go from fully flooded to semi-flooded, and the coin cell still maintains function until (AE/S)min ≈ 1.2 µL mg-1 is reached. When scaled up to Ah-level pouch cells, the full-cell energy density can reach 481 Wh kg-1 as its E/S ≈ AE/S ratio can be reduced to 1.2 µL mg-1, proving high-performance pouch cells can actually be working in the ultralean, semi-flooded regime.

Operando Analysis of a Lithium/Sulfur Battery by Small-Angle Neutron Scattering

This study reports the use of operando small-angle neutron scattering to investigate processes in an operating Li/S battery. The combination with impedance spectroscopy yields valuable insights into the precipitation and dissolution of lithium sulfide during 10 cycles of galvanostatic cycling. The use of a deuterated electrolyte increases strongly the sensitivity to detect the sulfur and Li₂S precipitates at the carbon host electrode and allows us to observe the time-dependent initial wetting of the system. No correlation of the scattering signal of the micropores with either lithium sulfide or sulfur is observable during the whole course of the experiment. Hence both reaction products do not precipitate inside the microporous structure but on the outer surface of the micrometer-sized carbon fibers used in this study. The excellent scattering contrast allows a detailed analysis of the formation and dissolution process of nanoscopic Li₂S structures. While lithium sulfide particles grow homogeneously during the precipitation period, smaller Li₂S particles dissolve first followed by a sudden dissolution of the larger Li₂S particles.

Self-Supported and Flexible Sulfur Cathode Enabled via Synergistic Confinement for High-Energy-Density Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries have attracted much attention in the field of electrochemical energy storage due to their high energy density and low cost. However, the "shuttle effect" of the sulfur cathode, resulting in poor cyclic performance, is a big barrier for the development of Li-S batteries. Herein, a novel sulfur cathode integrating sulfur, flexible carbon cloth, and metal-organic framework (MOF)-derived N-doped carbon nanoarrays with embedded CoP (CC@CoP/C) is designed. These unique flexible nanoarrays with embedded polar CoP nanoparticles not only offer enough voids for volume expansion to maintain the structural stability during the electrochemical process, but also promote the physical encapsulation and chemical entrapment of all sulfur species. Such designed CC@CoP/C cathodes with synergistic confinement (physical adsorption and chemical interactions) for soluble intermediate lithium polysulfides possess high sulfur loadings (as high as 4.17 mg cm^-2 ) and exhibit large specific capacities at different C-rates. Specially, an outstanding long-term cycling performance can be reached. For example, an ultralow decay of 0.016% per cycle during the whole 600 cycles at a high current density of 2C is displayed. The current work provides a promising design strategy for high-energy-density Li-S batteries.

Electrocatalysis of Ruthenium Nanoparticles-Decorated Hollow Carbon Spheres for the Conversion of Li2S2/Li2S in Lithium-Sulfur Batteries

Controlling "polysulfide dissolution" and pacifying "polysulfide shuttle" hold the key in developing a lithium-sulfur battery with superior electrochemical performance. Further, exploration of the concept of electrocatalysts plays a significant role in enhancing the electrochemical reversibility of polysulfides in lithium-sulfur battery. Herein, ruthenium nanoparticles-decorated porous, hollow carbon spheres have been successfully prepared and deployed as electrocatalyst as well as sulfur host in the lithium-sulfur battery assembly. Interaction of sulfur with ruthenium nanoparticles has been explained with appropriate electroanalytical and electrochemical characterization techniques. We observe that lithium-sulfur battery containing C-Ru-S cathode with a fixed sulfur loading exhibits a significantly improved capacity of 1200 mA h g^-1 at C/10 current rate for 100 cycles. Volume expansion-related issues are found to get addressed by the hollow structured carbon spheres, and the electrocatalytic activity will improve the reaction kinetics of the conversion of Li₂S₂ to Li₂S and vice versa.