Recent Progress in Liquid Electrolyte-Based Li–S Batteries: Shuttle Problem and Solutions

A carbon quantum dot-decorated g-C3N4 composite as a sulfur hosting material for lithium-sulfur batteries

Although lithium-sulfur (Li-S) batteries have attracted strong consideration regarding their fundamental mechanism and energy applications, the inferior cycling performance and low reaction rate caused by the "shuttling effect" and the sluggish reaction kinetics of lithium polysulfides (LiPSs) impede their practical application. In this work, graphitic C3N4 (g-C3N4) assembled with highly-dispersed nitrogen-containing carbon quantum dots (CQDs) is designed as a cooperative catalyst to accelerate the reaction kinetics of LiPS conversion, the precipitation of Li2S during discharging, and insoluble Li2S decomposition during the charging process. Meanwhile, the introduction of CQDs improves the conductivity of the g-C3N4 substrate, showing great significance for the construction of high-performance electrocatalysts. As a result, the as-obtained composite shows efficient adsorption and electrochemical conversion of LiPSs, and the Li-S batteries assembled with CQDs/g-C3N4 exhibit an initial specific capacity of 1300.0 mA h g-1 at the current density of 0.1C and retain 582.3 mA h g-1 after 200 cycles. The electrode with the modified composite displays a greater capacity contribution of Li2S precipitation (175.7 mA h g-1), indicating an enhanced catalytic activity of g-C3N4 decorated by CQDs. The rational design of CQDs/g-C3N4 as a sulfur host could be an effective strategy for developing high performance Li-S batteries.

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.

Solid-State Electrolytes in Lithium-Sulfur Batteries: Latest Progresses and Prospects

Solid-state lithium-sulfur batteries (SSLSBs) have attracted tremendous research interest due to their large theoretical energy density and high safety, which are highly important indicators for the development of next-generation energy storage devices. Particularly, safety and "shuttle effect" issues originating from volatile and flammable liquid organic electrolytes can be fully mitigated by switching to a solid-state configuration. However, their road to thecommercial application is still plagued with numerous challenges, most notably the intrinsic electrochemical instability of solid-state electrolytes (SSEs) materials and their interfacial compatibility with electrodes and electrolytes. In this review, a critical discussion on the key issues and problems of different types of SSEs as well as the corresponding optimization strategies are first highlighted. Then, the state-of-the-art preparation methods and properties of different kinds of SSE materials, and their manufacture, characterization and performance in SSLSBs are summarized in detail. Finally, a scientific outlook for the future development of SSEs and the avenue to commercial application of SSLSBs is also proposed.

A hybrid ionic liquid-based electrolyte for high-performance lithium–sulfur batteries

A hybrid IL-based electrolyte consisting of P_1,2O1TFSI and the support solvent(s) of DOL and/or ETFE was applied in Li–S batteries, and a rational balance between Li₂S_x dissolution and Li protection to achieve controllable shuttle was proposed.

Elaboration of Aggregated Polysulfide Phases: From Molecules to Large Clusters and Solid Phases

With the increasing strategies aimed at repressing shuttle problems in the lithium-sulfur battery, dissolved contents of polysulfides are significantly reduced. Except for solid-state Li₂S₂ and Li₂S, aggregated phases of polysulfides remain unexplored, especially in well confined cathode material systems. Here, we report a series of nanosize polysulfide clusters and solid phases from an atomic perspective. The calculated phase diagram and formation energy evolution process demonstrate their stabilities and cohesive tendency. It is interesting to find that Li₂S₆ can stay in the solid state and contains short S₃ chains, further leading to the unique stability and dense structure. Simulated electronic properties indicate reduced band gaps when polysulfides are aggregated, especially for solid phase Li₂S₆ with a band gap as low as 0.47 eV. Their dissolution behavior and conversion process are also investigated, which provides a more realistic model and gives further suggestions on the future design of the lithium-sulfur battery.

Current Status and Future Prospects of Metal-Sulfur Batteries

Lithium-sulfur batteries are a major focus of academic and industrial energy-storage research due to their high theoretical energy density and the use of low-cost materials. The high energy density results from the conversion mechanism that lithium-sulfur cells utilize. The sulfur cathode, being naturally abundant and environmentally friendly, makes lithium-sulfur batteries a potential next-generation energy-storage technology. The current state of the research indicates that lithium-sulfur cells are now at the point of transitioning from laboratory-scale devices to a more practical energy-storage application. Based on similar electrochemical conversion reactions, the low-cost sulfur cathode can be coupled with a wide range of metallic anodes, such as sodium, potassium, magnesium, calcium, and aluminum. These new "metal-sulfur" systems exhibit great potential in either lowering the production cost or producing high energy density. Inspired by the rapid development of lithium-sulfur batteries and the prospect of metal-sulfur cells, here, over 450 research articles are summarized to analyze the research progress and explore the electrochemical characteristics, cell-assembly parameters, cell-testing conditions, and materials design. In addition to highlighting the current research progress, the possible future areas of research which are needed to bring conversion-type lithium-sulfur and other metal-sulfur batteries into the market are also discussed.

Promoting the Transformation of Li2 S2 to Li2 S: Significantly Increasing Utilization of Active Materials for High-Sulfur-Loading Li-S Batteries

Lithium-sulfur (Li-S) batteries with high sulfur loading are urgently required in order to take advantage of their high theoretical energy density. Ether-based Li-S batteries involve sophisticated multistep solid-liquid-solid-solid electrochemical reaction mechanisms. Recently, studies on Li-S batteries have widely focused on the initial solid (sulfur)-liquid (soluble polysulfide)-solid (Li₂ S₂ ) conversion reactions, which contribute to the first 50% of the theoretical capacity of the Li-S batteries. Nonetheless, the sluggish kinetics of the solid-solid conversion from solid-state intermediate product Li₂ S₂ to the final discharge product Li₂ S (corresponding to the last 50% of the theoretical capacity) leads to the premature end of discharge, resulting in low discharge capacity output and low sulfur utilization. To tackle the aforementioned issue, a catalyst of amorphous cobalt sulfide (CoS₃ ) is proposed to decrease the dissociation energy of Li₂ S₂ and propel the electrochemical transformation of Li₂ S₂ to Li₂ S. The CoS₃ catalyst plays a critical role in improving the sulfur utilization, especially in high-loading sulfur cathodes (3-10 mg cm^-2 ). Accordingly, the Li₂ S/Li₂ S₂ ratio in the discharge products increased to 5.60/1 from 1/1.63 with CoS₃ catalyst, resulting in a sulfur utilization increase of 20% (335 mAh g^-1 ) compared to the counterpart sulfur electrode without CoS₃ .

Surface-Modified Sulfur Nanorods Immobilized on Radially Assembled Open-Porous Graphene Microspheres for Lithium-Sulfur Batteries

The assembly of two-dimensional conductive nanomaterials into hierarchical complex architectures precisely controlling internal open porosity and orientation, external morphology, composition, and interaction is expected to provide promising hosts for high-capacity sulfur cathodes. Herein, we demonstrate rod-like nanosulfur (nS) deposited onto radially oriented open-porous microspherical reduced graphene oxide (rGO) architectures for improved rate and cyclic capabilities of lithium-sulfur (Li-S) batteries. The combined chemistry of a spray-frozen assembly and ozonation drives the formation of a radially oriented open-porous structure and an overall microspherical morphology as well as uniform distribution and high loading of rod-like nS. Moreover, an optimum composition and strong bonding of the rGO/nS hybrid enables the optimization of redox kinetics for high sulfur utilization and high-rate capacities. The resulting rGO/nS hybrid provides a specific capacity and first-cycle Coulombic efficiency of 1269.1 mAh g^-1 and 98.5%, respectively, which are much greater than those of ice-templated and physically mixed rGO/nS hybrids and radially oriented open-porous rGO/bulk sulfur with the same hybrid composition. A 4C capacity of 510.3 mAhg^-1 and capacity decay of 0.08% per cycle over 500 cycles (70.9% of the initial capacity over 300 cycles) also support the synergistic effect of the rod-like nS strongly interacting with the radially oriented open-porous rGO microspheres.

High-Performance Li-SeSx All-Solid-State Lithium Batteries

All-solid-state Li-S batteries are promising candidates for next-generation energy-storage systems considering their high energy density and high safety. However, their development is hindered by the sluggish electrochemical kinetics and low S utilization due to high interfacial resistance and the electronic insulating nature of S. Herein, Se is introduced into S cathodes by forming SeS_x solid solutions to modify the electronic and ionic conductivities and ultimately enhance cathode utilization in all-solid-state lithium batteries (ASSLBs). Theoretical calculations confirm the redistribution of electron densities after introducing Se. The interfacial ionic conductivities of all achieved SeS_x -Li₃ PS₄ (x = 3, 2, 1, and 0.33) composites are 10^-6 S cm^-1 . Stable and highly reversible SeS_x cathodes for sulfide-based ASSLBs can be developed. Surprisingly, the SeS₂ /Li₁₀ GeP₂ S₁₂ -Li₃ PS₄ /Li solid-state cells exhibit excellent performance and deliver a high capacity over 1100 mAh g^-1 (98.5% of its theoretical capacity) at 50 mA g^-1 and remained highly stable for 100 cycles. Moreover, high loading cells can achieve high areal capacities up to 12.6 mAh cm^-2 . This research deepens the understanding of Se-S solid solution chemistry in ASSLB systems and offers a new strategy to achieve high-performance S-based cathodes for application in ASSLBs.