Review of Cathode in Advanced Li−S Batteries: The Effect of Doping Atoms at Micro Levels

Electrochemical coverage of reduced graphene oxide layers on sulfur supported by biochar for enhancing performance of Li-S battery

To improve the electrochemical performance of Li-S batteries, a cathodic material (rGO150/S/CF-75) was fabricated for Li-S batteries by adopting a melt-flow method to load sulfur on biomass-derived carbon fibers, then the reduced graphene oxide was electrochemically covered on the outside surface of the sulfur. The coverage of reduced graphite oxide layers endows the performance of S/CF-75 multiple improvements. The specific capacity of rGO150/S/CF-75 cathode delivers a specific capacity of 1451.4 mAh g-1 at 0.1 A g-1. The specific capacity of rGO150/S/CF-75 cathode can still maintain 537.3 mAh g-1 after 1000 cycles at 5 A g-1 (109 % capacity retention). The excellent performance of rGO150/S/CF-75 cathode is benefit from not only the conductive paths of reduced graphene oxide layers and protective function of reduced graphene oxide layers inhibiting that the soluble sulfur diffuse into bulk electrolyte, but also the redistribution of sulfur on conductive carbon components during the cycling process.

Kinetic Promoters for Sulfur Cathodes in Lithium-Sulfur Batteries

ConspectusLithium-sulfur (Li-S) batteries have attracted worldwide attention as promising next-generation rechargeable batteries due to their high theoretical energy density of 2600 Wh kg-1. The actual energy density of Li-S batteries at the pouch cell level has significantly exceeded that of state-of-the-art Li-ion batteries. However, the overall performances of Li-S batteries under practical working conditions are limited by the sluggish conversion kinetics of the sulfur cathodes. To overcome the above challenge, various kinetic promotion strategies have been proposed to accelerate the multiphase and multi-electron cathodic redox reactions between sulfur, lithium polysulfides (LiPSs), and lithium sulfide. Nowadays, kinetic promoters have been massively employed in sulfur cathodes to achieve Li-S batteries with high energy densities, high rates, and long lifespans. A comprehensive and timely summary of cutting-edge kinetic promoters for sulfur cathodes is of great essence to afford an in-depth understanding of the unique Li-S electrochemistry.In this Account, we outline the recent efforts on the design of sulfur cathode kinetic promoters for advanced Li-S batteries. The latest progress is discussed in detail regarding heterogeneous, homogeneous, and semi-immobilized kinetic promoters. Heterogeneous promoters, representatively known as electrocatalysts, function mainly by reducing the energy barriers of the interfacial electrochemical reactions. The working mechanism, activity regulation strategies, and reconstitution/deactivation processes of the heterogeneous promoters are reviewed to provide guiding principles for rational design. In comparison, homogeneous promoters are able to fully contact with the reaction interfaces and regulate the electron/ion-inaccessible reactants in working Li-S batteries. Redox mediators and redox comediators are typical homogeneous promoters. The former establishes extra chemical reaction pathways to circumvent the originally sluggish steps and boost the overall kinetics, while the latter fundamentally modifies the LiPS molecules to enhance their redox kinetics. For semi-immobilized promoters, the active units are generally anchored on the cathode substrate through flexible chains with mobile characteristics. Such a design endows the promoter with both heterogeneous and homogeneous characteristics to comprehensively regulate the multiphase sulfur redox reactions involving both mobile and immobile reactants.Overall, this Account summarizes the fundamental electrochemistry, design principles, and practical promotion effects of the various kinetic promoters used for the sulfur cathodes in Li-S batteries. We believe that this Account will provide an in-depth and cutting-edge understanding of the unique sulfur electrochemistry, thereby providing guidance for further development of high-performance Li-S batteries and analogous rechargeable battery systems.

Biomass Separators as a "Lifesaver" for Safe and Long-Life Lithium Metal Batteries

The growth of lithium dendrites and the shuttle of polysulfides in lithium metal batteries (LMBs) have hindered their development. In LMBs, the cathode and anode are separated by a separator, although this does not solve the battery's issues. The use of biomass materials is widespread for modifying the separator due to their porous structure and abundant functional groups. LMBs perform more electrochemically when lithium ions are deposited uniformly and polysulfide shuttling is reduced using biomass separators. In this review, we analyze the growth of lithium dendrite and the shuttle of polysulfide in LMBs, summarize the types of biomass separator materials and the mechanisms of action (providing mechanical barriers, promoting uniform deposition of metal ions, capturing polysulfides, shielding polysulfide). The prospect of developing new separator materials from the perspective of regulating ion transport and physical sieving efficiency as well as the application of advanced technologies such as synchrotron radiation to characterize the mechanism of action of biomass separators is also proposed.

Dual-Defect Engineering of Bidirectional Catalyst for High-Performing Lithium-Sulfur Batteries

Practical applications of lithium-sulfur (Li-S) batteries have been hindered by sluggish reaction kinetics and severe capacity decay during charge-discharge cycling due to the notorious shuttle effect of polysulfide and the unfavored deposition and dissolution of Li2 S. Herein, to address these issues, a double-defect engineering strategy is developed for preparing Co-doped FeP catalyst containing P vacancies on MXene, which effectively improves the bidirectional redox of Li2 S. Mechanism analysis indicates that P vacancy accelerates Li2 S nucleation via increased unsaturated sites, and Co doping generates local electric field to reduce the reaction energy barrier and accelerate Li2 S dissolution. MXene provides highly conductive channels for electron transport, and effectively captures polysulfide. The double-defect catalyst enables an impressive reversible specific capacity of 1297.9 mAh g-1 at 0.2 C, and excellent rate capability of 726.5 mAh g-1 at 4 C. Remarkably, it demonstrates excellent cycling stability with capacity retention of 533.3 mAh g-1 after 500 cycles at 2 C. The results can unlock the double-defect engineering of vacancy induction and heteroatomic doping towards practical Li-S batteries.