Sulfur Confined in Sub-Nanometer-Sized 2 D Graphene Interlayers and Its Electrochemical Behavior in Lithium-Sulfur Batteries

Electrochemical coupling in subnanometer pores/channels for rechargeable batteries

Subnanometer pores/channels (SNPCs) play crucial roles in regulating electrochemical redox reactions for rechargeable batteries. The delicately designed and tailored porous structure of SNPCs not only provides ample space for ion storage but also facilitates efficient ion diffusion within the electrodes in batteries, which can greatly improve the electrochemical performance. However, due to current technological limitations, it is challenging to synthesize and control the quality, storage, and transport of nanopores at the subnanometer scale, as well as to understand the relationship between SNPCs and performances. In this review, we systematically classify and summarize materials with SNPCs from a structural perspective, dividing them into one-dimensional (1D) SNPCs, two-dimensional (2D) SNPCs, and three-dimensional (3D) SNPCs. We also unveil the unique physicochemical properties of SNPCs and analyse electrochemical couplings in SNPCs for rechargeable batteries, including cathodes, anodes, electrolytes, and functional materials. Finally, we discuss the challenges that SNPCs may face in electrochemical reactions in batteries and propose future research directions.

The "dual-layer sulfur cathode" strategy: An In2S3/Bi2S3@rGO heterostructure as an interlayer/modified separator for boosting the areal capacities of lithium-sulfur batteries

The specific energies and energy densities of lithium-sulfur (Li-S) batteries are influenced by various cell parameters, including the sulfur loading, the sulfur weight percentage in the cathode, and the electrolyte/sulfur ratio. An In2S3/Bi2S3@rGO heterostructure was obtained by growing indium sulfide nanoparticles on the surface of bismuth sulfide nanoflowers in a graphene oxide (GO) solution via a one-step solvothermal approach. This structure was introduced as a modified separator/dual-layer sulfur cathode for Li-S batteries. The Bi2S3/In2S3 heterointerfaces act as active sites to speed up interfacial electron transfer, along with the entrapment, diffusion, and transformation of lithium polysulfides. A Li-S cell containing a dual-layer sulfur cathode (thin layer of In2S3/Bi2S3@rGO sandwiched between two thick layers of sulfur) and coupled with an In2S3/Bi2S3@rGO-coated separator suppressed the polysulfide shuttle effect. The cell based on the dual-layer sulfur cathode technology and operated at a current rate of 0.3C achieved a high capacity (7.1 mAh cm-2) after the 200th cycle, giving an electrolyte/sulfur ratio (10 µL mg-1) under a high sulfur loading (11.53 mg cm-2). These results demonstrate the unique nature of the dual-layer sulfur cathode technique, which can yield high energy density Li-S batteries with high sulfur loadings and low electrolyte/sulfur ratios.

Polyethylenimine Expanded Graphite Oxide Enables High Sulfur Loading and Long-Term Stability of Lithium-Sulfur Batteries

To realize practical lithium-sulfur batteries (LSBs) with long cycling life, designing cathode hosts with a high specific surface area (SSA) is recognized as an efficient way to trap the soluble polysulfides. However, it is also blamed for diminishing the volumetric energy density and being susceptible to side reactions. Herein, polyethylenimine intercalated graphite oxide (PEI-GO) with a low SSA of 4.6 m² g^-1 and enlarged interlayer spacing of 13 Å is proposed as a superior sulfur host, which enables homogeneous distribution of high sulfur content (73%) and facilitates Li⁺ transfer in thick sulfur electrode. LSBs with a moderate sulfur loading (3.4 mg S cm^-2 ) achieve an initial capacity of 1157 and 668 mAh g^-1 after 500 cycles at 0.5 C. Even when the sulfur loading is increased to 7.3 mg cm^-2 , the electrode still delivers a high areal capacity of 4.7 mAh cm^-2 (641 mAh g^-1 ) after 200 cycles at 0.2 C. The excellent electrochemical properties of PEI-GO are mainly attributed to the homogeneous distribution of sulfur in PEI-GO and the strong chemical interactions between polysulfides and amine groups, which can mitigate the loss of active phases and contribute to the better cycling stability.

Structural Design of Lithium–Sulfur Batteries: From Fundamental Research to Practical Application

Abstract

Lithium–sulfur (Li–S) batteries have been considered as one of the most promising energy storage devices that have the potential to deliver energy densities that supersede that of state-of-the-art lithium ion batteries. Due to their high theoretical energy density and cost-effectiveness, Li–S batteries have received great attention and have made great progress in the last few years. However, the insurmountable gap between fundamental research and practical application is still a major stumbling block that has hindered the commercialization of Li–S batteries. This review provides insight from an engineering point of view to discuss the reasonable structural design and parameters for the application of Li–S batteries. Firstly, a systematic analysis of various parameters (sulfur loading, electrolyte/sulfur (E/S) ratio, discharge capacity, discharge voltage, Li excess percentage, sulfur content, etc.) that influence the gravimetric energy density, volumetric energy density and cost is investigated. Through comparing and analyzing the statistical information collected from recent Li–S publications to find the shortcomings of Li–S technology, we supply potential strategies aimed at addressing the major issues that are still needed to be overcome. Finally, potential future directions and prospects in the engineering of Li–S batteries are discussed.

Graphical Abstract

Effective Cathode Design of Three-Layered Configuration for High-Energy Li-S Batteries

A three-layered cathode structure was designed to minimize the shuttle effect of polysulfides and improve active material utilization. The three-layered configuration was fabricated by directly dropping pure sulfur composite slurry into multifunctional dual-barrier layers consisting of a self-standing TiO₂/C interlayer and a very thin acetylene black layer (0.35 mg cm^-2). In consequence, a decent discharge capacity of 963 mA h g^-1 was acquired after 100 cycles at 0.1 C. With cycling at 0.1, 0.2, 0.5, 1, and 2 C, the cells displayed excellent reversible capacities of 1203, 1145, 1035, 934, and 820 mA h g^-1, respectively. Furthermore, the cells still delivered a satisfactory discharge capacity of 799 mA h g^-1 after 300 cycles at 0.5 C. The light mass of the three-layered configuration guarantees that the energy density is effectively improved, considering the overall mass of the cathode. The energy density (603 W h kg^-1 after 100 cycles) was at a high level compared with those of the reported ones. Therefore, it is believed that the synergistic design for the three-layered cathode structure, which combines the mass-produced layer-by-layer structure, provides a novel protocol to the practical application of lithium-sulfur batteries.

More Reliable Lithium-Sulfur Batteries: Status, Solutions and Prospects

Lithium-sulfur (Li-S) batteries have attracted tremendous interest because of their high theoretical energy density and cost effectiveness. The target of Li-S battery research is to produce batteries with a high useful energy density that at least outperforms state-of-the-art lithium-ion batteries. However, due to an intrinsic gap between fundamental research and practical applications, the outstanding electrochemical results obtained in most Li-S battery studies indeed correspond to low useful energy densities and are not really suitable for practical requirements. The Li-S battery is a complex device and its useful energy density is determined by a number of design parameters, most of which are often ignored, leading to the failure to meet commercial requirements. The purpose of this review is to discuss how to pave the way for reliable Li-S batteries. First, the current research status of Li-S batteries is briefly reviewed based on statistical information obtained from literature. This includes an analysis of how the various parameters influence the useful energy density and a summary of existing problems in the current Li-S battery research. Possible solutions and some concerns regarding the construction of reliable Li-S batteries are comprehensively discussed. Finally, insights are offered on the future directions and prospects in Li-S battery field.

Optimization of Microporous Carbon Structures for Lithium-Sulfur Battery Applications in Carbonate-Based Electrolyte

Developing appropriate sulfur cathode materials in carbonate-based electrolyte is an important research subject for lithium-sulfur batteries. Although several microporous carbon materials as host for sulfur reveal the effect, methods for producing microporous carbon are neither easy nor well controllable. Moreover, due to the complexity and limitation of microporous carbon in their fabrication process, there has been rare investigation of influence on electrochemical behavior in the carbonate-based electrolyte for lithium-sulfur batteries by tuning different micropore size(0-2 nm) of carbon host. Here, we demonstrate an immediate carbonization process, self-activation strategy, which can produce microporous carbon for a sulfur host from alkali-complexes. Besides, by changing different alkali-ion in the previous complex, the obtained microporous carbon exhibits a major portion of ultramicropore (<0.7 nm, from 54.9% to 25.8%) and it is demonstrated that the micropore structure of the host material plays a vital role in confining sulfur molecule. When evaluated as cathode materials in a carbonate-based electrolyte for Li-S batteries, such microporous carbon/sulfur composite can provide high reversible capacity, cycling stability and good rate capability.

Immobilizing Polysulfides with MXene-Functionalized Separators for Stable Lithium-Sulfur Batteries

Lithium-sulfur batteries have attracted increasing attention as one of the most promising candidates for next-generation energy storage systems. However, the poor cycling performance and the low utilization of sulfur greatly hinder its practical applications. Here we report the improved performance of lithium-sulfur batteries by coating Ti₃C₂T_x MXene nanosheets (where T stands for the surface termination, such as -O, -OH, and/or -F) on commercial "Celgard" membrane. In favor of the ultrathin two-dimensional structure, the Ti₃C₂T_x MXene can form a uniform coating layer with a minimum mass loading of 0.1 mg cm^-2 and a thickness of only 522 nm. Owing to the improved electric conductivity and the effective trapping of polysulfides, the lithium-sulfur batteries with MXene-functionalized separators exhibit superior performance including high specific capacities and cycling stability.