Understanding the Redox Obstacles in High Sulfur-Loading Li-S Batteries and Design of an Advanced Gel Cathode

Early Failure of Lithium-Sulfur Batteries at Practical Conditions: Crosstalk between Sulfur Cathode and Lithium Anode

Lithium-sulfur (Li-S) batteries are one of the most promising next-generation energy storage technologies due to their high theoretical energy and low cost. However, Li-S cells with practically high energy still suffer from a very limited cycle life with reasons which remain unclear. Here, through cell study under practical conditions, it is proved that an internal short circuit (ISC) is a root cause of early cell failure and is ascribed to the crosstalk between the S cathode and Li anode. The cathode topography affects S reactions through influencing the local resistance and electrolyte distribution, particularly under lean electrolyte conditions. The inhomogeneous reactions of S cathodes are easily mirrored by the Li anodes, resulting in exaggerated localized Li plating/stripping, Li filament formation, and eventually cell ISC. Manipulating cathode topography is proven effective to extend the cell cycle life under practical conditions. The findings of this work shed new light on the electrode design for extending cycle life of high-energy Li-S cells, which are also applicable for other rechargeable Li or metal batteries.

Enhanced Catalysis of LIS3· Radical-to-Polysulfide Interconversion via Increased Sulfur Vacancies in Lithium–Sulfur Batteries

The practical application of lithium–sulfur (Li–S) batteries is seriously hindered by severe lithium polysulfide (LiPS) shuttling and sluggish electrochemical conversions. Herein, the Co₉S₈/MoS₂ heterojunction as a model cathode host material is employed to discuss the performance improvement strategy and elucidate the catalytic mechanism. The introduction of sulfur vacancies can harmonize the chemisorption of the heterojunction component. Also, sulfur vacancies induce the generation of radicals, which participate in a liquidus disproportionated reaction to reduce the accumulation of liquid LiPSs. To assess the conversion efficiency from liquid LiPSs to solid Li₂S, a new descriptor calculated from basic cyclic voltammetry curves, nucleation transformation ratio, is proposed.

Sulfur vacancies can harmonize chemisorption and generate amounts of radicals to boost the conversions of LiPSs.

A Rational Reconfiguration of Electrolyte for High-Energy and Long-Life Lithium-Chalcogen Batteries

Lithium-chalcogen batteries are an appealing choice for high-energy-storage technology. However, the traditional battery that employs liquid electrolytes suffers irreversible loss and shuttle of the soluble intermediates. New batteries that adopt Li⁺ -conductive polymer electrolytes to mitigate the shuttle problem are hindered by incomplete discharge of sulfur/selenium. To address the trade-off between energy and cycle life, a new electrolyte is proposed that reconciles the merits of liquid and polymer electrolytes while resolving each of their inferiorities. An in situ interfacial polymerization strategy is developed to create a liquid/polymer hybrid electrolyte between a LiPF₆ -coated separator and the cathode. A polymer-gel electrolyte in situ formed on the separator shows high Li⁺ transfer number to serve as a chemical barrier against the shuttle effect. Between the gel electrolyte and the cathode surface is a thin gradient solidification layer that enables transformation from gel to liquid so that the liquid electrolyte is maintained inside the cathode for rapid Li⁺ transport and high utilization of active materials. By addressing the dilemma between the shuttle chemistry and incomplete discharge of S/Se, the new electrolyte configuration demonstrates its feasibility to trigger higher capacity retention of the cathodes. As a result, Li-S and Li-Se cells with high energy and long cycle lives are realized, showing promise for practical use.

Sulfur Diffusion within Nitrogen-Doped Ordered Mesoporous Carbons Determined by in Situ X-ray Scattering

The low intrinsic conductivity of sulfur necessitates conductive additives, such as mesoporous carbons, to the cathode to enable high-performance metal-sulfur batteries. Simultaneous efforts to address polysulfide shuttling have introduced nitrogen-doped carbons to provide both conductivity and suppressed shuttling because of their strong interaction with sulfur. The strength of this interaction will impact the ability to fill the mesopores with sulfur via melt infusion. Here, we systematically investigate how nitrogen doping influences the rate that molten sulfur can infiltrate the mesopores and the overall extent of pore filling of highly ordered mesoporous doped carbons using in situ small angle X-ray scattering (SAXS). The similarity in electron density between molten sulfur and the soft carbon framework of the mesoporous material leads to a precipitous decrease in the scattered intensity associated with the ordered structure as voids are filled with sulfur. As the nitrogen doping increases from 1 to 20 at. %, the effective diffusivity of sulfur in the mesopores decreases by an order of magnitude (2.7 × 10^-8 to 2.3 × 10^-9 cm/s). The scattering becomes nearly invariant within 20 min of melt infiltration at 155 °C for all but the most doped carbon, which indicates that submicron-sized mesoporous carbon particles can be filled rapidly. Additionally, the nitrogen doping decreases the sulfur content that can be accommodated within the mesopores from 95% of the mesopores filled without doping to only 64% filled with 20 at. % N as determined by the residual scattering intensity. Sulfur does not crystallize within the mesopores of the nitrogen-doped carbons, which is further indicative of the strong interactions between the nitrogen species and sulfur that can inhibit polysulfide shuttling. In situ SAXS provides insights into the diffusion of sulfur in mesopores and how the surface chemistry of nitrogen-doped carbon appears to significantly hinder the infiltration by sulfur.

A Polysulfide-Infiltrated Carbon Cloth Cathode for High-Performance Flexible Lithium-Sulfur Batteries

For practical application of lithium-sulfur batteries (LSBs), it is crucial to develop sulfur cathodes with high areal capacity and cycle stability in a simple and inexpensive manner. In this study, a carbon cloth infiltrated with a sulfur-containing electrolyte solution (CC-S) was utilized as an additive-free, flexible, high-sulfur-loading cathode. A freestanding carbon cloth performed double duty as a current collector and a sulfur-supporting/trapping material. The active material in the form of Li₂S₆ dissolved in a 1 M LiTFSI-DOL/DME solution was simply infiltrated into the carbon cloth (CC) during cell fabrication, and its optimal loading amount was found to be in a range between 2 and 10 mg/cm² via electrochemical characterization. It was found that the interwoven carbon microfibers retained structural integrity against volume expansion/contraction and that the embedded uniform micropores enabled a high loading and an efficient trapping of sulfur species during cycling. The LSB coin cell employing the CC-S electrode with an areal sulfur loading of 6 mg/cm² exhibited a high areal capacity of 4.3 and 3.2 mAh/cm² at C/10 for 145 cycles and C/3 for 200 cycles, respectively, with minor capacity loss (<0.03%/cycle). More importantly, such high performance could also be realized in flexible pouch cells with dimensions of 2 cm × 6 cm before and after 300 bending cycles. Simple and inexpensive preparation of sulfur cathodes using CC-S electrodes, therefore, has great potential for the manufacture of high-performance flexible LSBs.

Sepiolite/CNT/S@PANI composite with stable network structure for high performance lithium sulfur batteries

Composite materials with a stable network structure consisting of natural sepiolite (Sep) powders, carbon nanotubes (CNTs) and conductive polymer (PANI) have been successfully synthesized using a simple vacuum heat treatment and chemical oxidation method, and they have been used as cathode materials for lithium sulfur batteries. It is found that Sep/CNT/S@PANI composites possess high initial discharge capacity, good cyclic stability and good rate performance. The initial discharge capacity of the Sep/CNT/S@PANI-II composite is about 1100 mA h g⁻¹ at 2C, and remained at 650 mA h g⁻¹ after 300 cycles, and the corresponding coulombic efficiency is above 93%. Such performance is attributed to specific porous structure, outstanding adsorption characteristics, and excellent ion exchange capability of sepiolite, as well as excellent conductivity of CNT. Furthermore, the PANI coating has a pinning effect for sulfur, which enhances the utilization of the active mass and improves the cycling stability and the coulombic efficiency of the composites at high current rates.

The cathode composite materials for lithium sulfur batteries with a stable network structure consisting of natural sepiolite powders, carbon nanotubes and conductive polymer were synthesized by vacuum heat treatment and chemical oxidation method.

Ion Speciation and Transport Properties of LiTFSI in 1,3-Dioxolane Solutions: A Case Study for Li-S Battery Applications

Lithium-sulfur battery is considered to be one of the main candidates for "post-lithium-ion" battery generation because of its high theoretical specific capacity and inherently low cost. The role of electrolyte is particularly important in this system, and remarkable battery performances have been reported by tuning the amount of salt in the electrolyte. To further understand the reasons for such improvements, we chose the lithium bis(trifluoromethanesulfonyl)imide in 1,3-dioxolane electrolyte as a model salt-solvent system for a systematic study of conductivity and viscosity over a wide range of concentration from 10^-5 up to 5 m. The experimental results, discussed and interpreted with reference to the theory of electrolyte conductance, lead to the conclusion that triple ion formation is responsible for the highest molal conductivity values before reaching the maximum at 1.25 m. At higher concentrations, the molal conductivity drops quickly because of a rapid increase in viscosity and the salt-solvent system can be treated as a diluted form of molten salt.

Integrated Design of MnO2 @Carbon Hollow Nanoboxes to Synergistically Encapsulate Polysulfides for Empowering Lithium Sulfur Batteries

Lithium sulfur batteries (LSBs) with high theoretical energy density are being pursued as highly promising next-generation large-scale energy storage devices. However, its launch into practical application is still shackled by various challenges. A rational nanostructure of hollow carbon nanoboxes filled with birnessite-type manganese oxide nanosheets (MnO₂ @HCB) as a new class of molecularly-designed physical and chemical trap for lithium polysulfides (Li₂ S_x (x = 4-8)) is reported. The bifunctional, integrated, hybrid nanoboxes overcome the obstacles of low sulfur loading, poor conductivity, and redox shuttle of LSBs via effective physical confinement and chemical interaction. Benefiting from the synergistic encapsulation, the developed MnO₂ @HCB/S hybrid nanoboxes with 67.9 wt% sulfur content deliver high specific capacity of 1042 mAh g^-1 at the current density of 1 A g^-1 with excellent Coulombic efficiency ≈100%, and retain improved reversible capacity during long term cycling at higher current densities. The developed strategy paves a new path for employing other metal oxides with unique architectures to boost the performance of LSBs.

Polysulfide intercalation in bilayer-structured graphitic C3N4: a first-principles study

A graphitic C₃N₄ bilayer can stabilize polysulfides (PSs) by using a micropore-scale interlayer as a promising anchoring material, and improve sulfur utilisation.