Zwitterionic Covalent Organic Framework Based Electrostatic Field Electrocatalysts for Durable Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) are one of the most promising candidates for next-generation energy storage systems. To develop long-life LSBs, there is an urgent need to develop functional materials with higher catalytic activity toward polysulfides and reduced dendritic lithium growth. Herein, an electrostatic field electrocatalyst is designed in a zwitterionic covalent organic framework (COF) with a "two birds with one stone" ability for simultaneously overcoming obstacles in the lithium metal anode and sulfur cathode. The synergism between cationic and anionic moieties in the zwitterionic COF creates an electrostatic field for bidirectionally catalyzing S cathode conversion. Besides, the rational design of zwitterionic COF as a separator modification layer allows lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissociation and fast lithium-ion conduction, which alleviates lithium dendrite growth and thus improves the cycling life of LSBs. This contribution not only pioneers the application of zwitterionic COF in the field of LSBs but also highlights the potential of electrostatic field electrocatalysts.

Cotton-Derived Fe/Fe3C-Encapsulated Carbon Nanotubes for High-Performance Lithium-Sulfur Batteries

Fabrication processes of fossil fuel-derived carbon nanomaterials are of high carbon emissions. Deriving carbon materials from low-cost and sustainable biomass is eco-friendly. Cotton, one of the most abundant biomass materials, naturally holds a hierarchically porous structure, making the activated cotton textile (ACT) an ideal scaffold for loading active materials. Here, we report a low-cost approach to massively producing multiwalled carbon nanotubes (MWCNTs) via a combination process of vapor-liquid-solid (VLS) and solid-liquid-solid (SLS) where cotton decomposed into carbon-containing gases and amorphous carbons that then dissolved into Fe nanoparticles, forming Fe/Fe₃C-encapsulated MWCNTs. The lithium-sulfur (Li-S) battery constructed by the Fe/Fe₃C-MWCNT@ACT/S composite (as the cathode) and the Fe/Fe₃C-MWCNT@ACT (as the interlayer) exhibited a superlative cycling stability (over 1000 cycles at 1.0 C), an ultralow capacity decay rate (0.0496% per cycle) and a remarkable specific capacity (1273 mAh g^-1 at 0.1 C). The Fe/Fe₃C-MWCNTs enhanced electrode stability and suppressed polysulfide dissolution during cycling.

Low Cost Metal Carbide Nanocrystals as Binding and Electrocatalytic Sites for High Performance Li-S Batteries

Lithium sulfur (Li-S) batteries are considered as promising energy storage systems for the next generation of batteries due to their high theoretical energy densities and low cost. Much effort has been made to improve the practical energy densities and cycling stability of Li-S batteries via diverse designs of materials nanostructure. However, achieving simultaneously good rate capabilities and stable cycling of Li-S batteries is still challenging. Herein, we propose a strategy to utilize a dual effect of metal carbide nanoparticles decorated on carbon nanofibers (MC NPs-CNFs) to realize high rate performance, low hysteresis, and long cycling stability of Li-S batteries in one system. The adsorption experiments of lithium polysulfides (LiPS) to MC NPs and corresponding theoretical calculations demonstrate that LiPS are likely to be adsorbed and diffused on the surface of MC NPs because of their moderate chemical bonding. MC NPs turn out to have also an electrocatalytic role and accelerate electrochemical redox reactions of LiPS, as proven by cyclic voltammetry analysis. The fabricated Li-S batteries based on the W₂C NPs-CNFs hybrid electrodes display not only high specific capacity of 1200 mAh/g at 0.2C but also excellent rate performance and cycling stability, for example, a model setup can be operated at 1C for 500 cycles maintaining a final specific capacity of 605 mAh/g with a degradation rate as low as 0.06%/cycle.

Three-Dimensionally Hierarchical Ni/Ni3S2/S Cathode for Lithium-Sulfur Battery

Lithium-sulfur (Li-S) batteries have attracted interest as a promising energy-storage technology due to their overwhelming advantages such as high energy density and low cost. However, their commercial success is impeded by deterioration of sulfur utilization, significant capacity fade, and poor cycle life, which are principally originated from the severe shuttle effect in relation to the dissolution and migration of lithium polysulfides. Herein, we proposed an effective and facile strategy to anchor the polysulfides and improve sulfur loading by constructing a three-dimensionally hierarchical Ni/Ni₃S₂/S cathode. This self-supported hybrid architecture is sequentially fabricated by the partial sulfurization of Ni foam by a mild hydrothermal process, followed by physical loading of elemental sulfur. The incorporation of Ni₃S₂, with high electronic conductivity and strong polysulfide adsorption capability, can not only empower the cathode to alleviate the shuttle effect, but also afford a favorable electrochemical environment with lower interfacial resistance, which could facilitate the redox kinetics of the anchored polysulfides. Consequently, the obtained Ni/Ni₃S₂/S cathode with a sulfur loading of ∼4.0 mg/cm² demonstrated excellent electrochemical characteristics. For example, at high current density of 4 mA/cm², this thick cathode demonstrated a discharge capacity of 441 mAh/g at the 150th cycle.

Three-Dimensional Growth of Li2S in Lithium-Sulfur Batteries Promoted by a Redox Mediator

During the discharge of a lithium-sulfur (Li-S) battery, an electronically insulating 2D layer of Li2S is electrodeposited onto the current collector. Once the current collector is enveloped, the overpotential of the cell increases, and its discharge is arrested, often before reaching the full capacity of the active material. Guided by a new computational platform known as the Electrolyte Genome, we advance and apply benzo[ghi]peryleneimide (BPI) as a redox mediator for the reduction of dissolved polysulfides to Li2S. With BPI present, we show that it is now possible to electrodeposit Li2S as porous, 3D deposits onto carbon current collectors during cell discharge. As a result, sulfur utilization improved 220% due to a 6-fold increase in Li2S formation. To understand the growth mechanism, electrodeposition of Li2S was carried out under both galvanostatic and potentiostatic control. The observed kinetics under potentiostatic control were modeled using modified Avrami phase transformation kinetics, which showed that BPI slows the impingement of insulating Li2S islands on carbon. Conceptually, the pairing of conductive carbons with BPI can be viewed as a vascular approach to the design of current collectors for energy storage devices: here, conductive carbon "arteries" dominate long-range electron transport, while BPI "capillaries" mediate short-range transport and electron transfer between the storage materials and the carbon electrode.

Highly Cyclable Lithium-Sulfur Batteries with a Dual-Type Sulfur Cathode and a Lithiated Si/SiOx Nanosphere Anode

Lithium-sulfur batteries could become an excellent alternative to replace the currently used lithium-ion batteries due to their higher energy density and lower production cost; however, commercialization of lithium-sulfur batteries has so far been limited due to the cyclability problems associated with both the sulfur cathode and the lithium-metal anode. Herein, we demonstrate a highly reliable lithium-sulfur battery showing cycle performance comparable to that of lithium-ion batteries; our design uses a highly reversible dual-type sulfur cathode (solid sulfur electrode and polysulfide catholyte) and a lithiated Si/SiOx nanosphere anode. Our lithium-sulfur cell shows superior battery performance in terms of high specific capacity, excellent charge-discharge efficiency, and remarkable cycle life, delivering a specific capacity of ∼750 mAh g(-1) over 500 cycles (85% of the initial capacity). These promising behaviors may arise from a synergistic effect of the enhanced electrochemical performance of the newly designed anode and the optimized layout of the cathode.