Highly Reversible Li-Se Batteries with Ultra-Lightweight N,S-Codoped Graphene Blocking Layer

High-Rate Lithium-Selenium Batteries Boosted by a Multifunctional Janus Separator Over a Wide Temperature Range of -30 °C to 60 °C

Lithium-selenium batteries are characterized by high volumetric capacity comparable to Li-S batteries, while ≈1025 times higher electrical conductivity of Se than S is favorable for high-rate capability. However, they also suffer from the "shuttling effect" of lithium polyselenides (LPSes) and Li dendrite growth. Herein, a multifunctional Janus separator is designed by coating hierarchical nitrogen-doped carbon nanocages (hNCNC) and AlN nanowires on two sides of commercial polypropylene (PP) separator to overcome these hindrances. At room temperature, the Li-Se batteries with the Janus separator exhibit an unprecedented high-rate capability (331 mAh g-1 at 25 C) and retain a high capacity of 408 mAh g-1 at 3 C after 500 cycles. Moreover, the high retained capacities are achieved over a wide temperature range from -30 °C to 60 °C, showing the potential application under extreme environments. The excellent performances result from the "1+1>2" synergism of suppressed LPSes shuttling by chemisorption and electrocatalysis of hNCNC on the cathode side and suppressed Li-dendrite growth by thermally conductive AlN-network on the anode side, which can be well understood by the "Bucket Effect". This Janus separator provides a general strategy to develop high-performance lithium-chalcogen (Se, S, SeS2 ) batteries.

Recent Advancements in Selenium-Based Cathode Materials for Lithium Batteries: A Mini-Review

Selenium (Se)-based cathode materials have garnered considerable interest for lithium-ion batteries due to their numerous advantages, including low cost, high volumetric capacity (3268 mAh cm−3), high density (4.82 g cm−3), ability to be cycled to high voltage (4.2 V) without failure, and environmental friendliness. However, they have low electrical conductivity, low coulombic efficiency, and polyselenide solubility in electrolytes (shuttle effect). These factors have an adverse effect on the electrochemical performance of Li-Se batteries, rendering them unsuitable for real-world use. In this study, we briefly examined numerous approaches to overcoming these obstacles, including selecting an adequate electrolyte, the composition of Se with carbonaceous materials, and the usage of metal selenide base electrodes. Furthermore, we examined the effect of introducing interlayers between the cathode and the separator. Finally, the remaining hurdles and potential study prospects in this expanding field are proposed to inspire further insightful work.

Review of Multifunctional Separators: Stabilizing the Cathode and the Anode for Alkali (Li, Na, and K) Metal-Sulfur and Selenium Batteries

Alkali metal batteries based on lithium, sodium, and potassium anodes and sulfur-based cathodes are regarded as key for next-generation energy storage due to their high theoretical energy and potential cost effectiveness. However, metal-sulfur batteries remain challenged by several factors, including polysulfides' (PSs) dissolution, sluggish sulfur redox kinetics at the cathode, and metallic dendrite growth at the anode. Functional separators and interlayers are an innovative approach to remedying these drawbacks. Here we critically review the state-of-the-art in separators/interlayers for cathode and anode protection, covering the Li-S and the emerging Na-S and K-S systems. The approaches for improving electrochemical performance may be categorized as one or a combination of the following: Immobilization of polysulfides (cathode); catalyzing sulfur redox kinetics (cathode); introduction of protective layers to serve as an artificial solid electrolyte interphase (SEI) (anode); and combined improvement in electrolyte wetting and homogenization of ion flux (anode and cathode). It is demonstrated that while the advances in Li-S are relatively mature, less progress has been made with Na-S and K-S due to the more challenging redox chemistry at the cathode and increased electrochemical instability at the anode. Throughout these sections there is a complementary discussion of functional separators for emerging alkali metal systems based on metal-selenium and the metal-selenium sulfide. The focus then shifts to interlayers and artificial SEI/cathode electrolyte interphase (CEI) layers employed to stabilize solid-state electrolytes (SSEs) in metal-sulfur solid-state batteries (SSBs). The discussion of SSEs focuses on inorganic electrolytes based on Li- and Na-based oxides and sulfides but also touches on some hybrid systems with an inorganic matrix and a minority polymer phase. The review then moves to practical considerations for functional separators, including scaleup issues and Li-S technoeconomics. The review concludes with an outlook section, where we discuss emerging mechanics, spectroscopy, and advanced electron microscopy (e.g. cryo-transmission electron microscopy (cryo-TEM) and cryo-focused ion beam (cryo-FIB))-based approaches for analysis of functional separator structure-battery electrochemical performance interrelations. Throughout the review we identify the outstanding open scientific and technological questions while providing recommendations for future research topics.

The Electrostatic Attraction and Catalytic Effect Enabled by Ionic-Covalent Organic Nanosheets on MXene for Separator Modification of Lithium-Sulfur Batteries

It is of great significance to mediate the redox kinetics and shuttle effect of polysulfides in pursuit of high-energy-density and long-life lithium-sulfur (Li-S) batteries. Herein, a new strategy is proposed based on the electrostatic attraction and catalytic effect of polysulfides for the modification of the polypropylene (PP) separator. Guanidinium-based ionic-covalent organic nanosheets (iCON) on the surface of Ti₃ C₂ is presented as a coating layer for the PP separator. The synergetic effects of Ti₃ C₂ and iCON provide new platforms to suppress the shuttle effect of polysulfides, expedite the redox kinetics of sulfur species, and promote efficient conversion of the intercepted polysulfides. The functional separator endows carbon nanotube/sulfur cathodes with excellent electrochemical performance. The average capacity decay per cycle within 2000 cycles at 2 C is as low as 0.006%. The separator is even effective in the case of sulfur content of 90 wt% and sulfur loading of 7.6 mg cm^-2 ; the reversible capacity, areal capacity, and volumetric capacity at 0.1 C are as high as 1186 mA h g^-1 , 9.01 mA h cm^-2 , and 1201 mA h cm^-3 , respectively. This work provides a promising approach toward separator modification for the development of high-performance Li-S batteries.

State-Of-The-Art and Future Challenges in High Energy Lithium-Selenium Batteries

Li-chalcogen batteries, especially the Li-S batteries (LSBs), have received paramount interests as next generation energy storage techniques because of their high theoretical energy densities. However, the associated challenges need to be overcome prior to their commercialization. Elemental selenium, another chalcogen member, would be an attractive alternative to sulfur owing to its higher electronic conductivity, comparable capacity density, and moreover, excellent compatibility with carbonate electrolytes. Unlike LSBs, the research and development of Li-Se batteries (LSeBs) have garnered burgeoning attention but are still in their infant stage, where a comprehensive yet in-depth overview is highly imperative to guide future research. Herein, a critical review of LSeBs, in terms of the underlying mechanisms, cathode design, blocking layer engineering, and emerging solid-state electrolytes is provided. First, the electrolyte-dependent electrochemistry of LSeBs is discussed. Second, the advances in Se-based cathodes are comprehensively summarized, especially highlighting the state-of-the-art Se_x S_y cathodes, and mainly focusing on their structures, compositions, and synthetic strategies. Third, the versatile separators/interlayers optimization and interface regulation are outlined, with a particular focus on the emerging solid-state electrolytes for advanced LSeBs. Last, the remaining challenges and research orientations in this booming field are proposed, which are expected to motivate more insightful works.

Boosting Redox-Active Sites of 1T MoS2 Phase by Phosphorus-Incorporated Hierarchical Graphene Architecture for Improved Li Storage Performances

Hybridizing and architecting two kinds of 2D nanomaterials are attractive for energy storage applications. Herein, the chemical and electronic coupling of redox active 1T MoS₂ phase with hierarchical phosphorus-doped graphene architecture (HMPGA) is accomplished by the strong interactions of 2D hybrid colloids. The spectroscopic analyses on the crystal structure, surface morphology, and composition confirm the efficient doping of phosphorus and the hybridization interaction of 1T MoS₂ with the phosphorus-incorporated graphene. The resulting HMPGA anode shows significant improvement in battery performances. The specific capacity is delivered to 1194 mAh g^-1 at 100 mA g^-1 with a cyclability of 93.3% over 600 cycles. This improvement is ascribed to the multicoupling effect arising from the abundant redox-actives sites of 1T MoS₂ phase boosted and stabilized by hierarchically architected, phosphorus-doped graphenes.

High-power lithium-selenium batteries enabled by atomic cobalt electrocatalyst in hollow carbon cathode

Selenium cathodes have attracted considerable attention due to high electronic conductivity and volumetric capacity comparable to sulphur cathodes. However, practical development of lithium-selenium batteries has been hindered by the low selenium reaction activity with lithium, high volume changes and rapid capacity fading caused by the shuttle effect of polyselenides. Recently, single atom catalysts have attracted extensive interests in electrochemical energy conversion and storage because of unique electronic and structural properties, maximum atom-utilization efficiency, and outstanding catalytic performances. In this work, we developed a facile route to synthesize cobalt single atoms/nitrogen-doped hollow porous carbon (Co_SA-HC). The cobalt single atoms can activate selenium reactivity and immobilize selenium and polyselenides. The as-prepared selenium-carbon (Se@Co_SA-HC) cathodes deliver a high discharge capacity, a superior rate capability, and excellent cycling stability with a Coulombic efficiency of ~100%. This work could open an avenue for achieving long cycle life and high-power lithium-selenium batteries.

Lithium selenium batteries are attractive energy storage systems, but they are hindered by low selenium reaction activity and rapid capacity fading. Herein, the authors report a selenium host with atomic cobalt electrocatalyst which exhibits superior performances in lithium-selenium batteries.

Template-Free Electrochemical Deposition of t-Se Nano- and Sub-micro Structures With Controlled Morphology and Dimensions

Selenium, depending on its crystal structure, can exhibit various properties and, as a result, be used in a wide range of applications. However, its exploitation has been limited due to the lack of understanding of its complex growth mechanism. In this work, template-free electrodeposition has been utilized for the first time to synthesize hexagonal-selenium (t-Se) microstructures of various morphologies at 80°C. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) revealed 5 reduction peaks, which were correlated with possible electrochemical or chemical reaction related to the formation of selenium. Potentiostatic electrodeposition using 100 mM SeO₂ showed selenium nanorods formed at−0.389 V then increased in diameter up to −0.490 V, while more negative potentials (-0.594 V) induced formation of sub-micron wires with average diameter of 708 ± 116 nm. Submicron tubes of average diameter 744 ± 130 nm were deposited at −0.696 V. Finally, a mixture of tubes, wires, and particles was observed at more cathodic potential due to a combination of nucleation, growth, dissolution of structures as well as formation of amorphous selenium via comproportionation reaction. Texture coefficient as a function of applied potential described the preferred orientation of the sub-microstructures changed from (100) direction to more randomly oriented as more cathodic potentials were applied. Lower selenium precursor concentration lead to formation of nanowires only with smaller average diameters (124 ± 42 nm using 1 mM, 153 ± 46 nm using 10 mM SeO₂ at −0.389 V). Time-dependent electrodeposition using 100 mM selenium precursor at −0.696 V explained selenium was formed first as amorphous, on top of which nucleation continued to form rods and wires, followed by preferential dissolution of the wire core to form tubes.

Selenium and nitrogen co-doped carbon quantum dots as a fluorescent probe for perfluorooctanoic acid

Highly fluorescent carbon quantum dots co-doped with selenium and nitrogen  (SeN-CQDs) were fabricated via a one-pot hydrothermal route using selenomethionine as the sole precursor. The SeN-CQDs aggregates have sizes between 30 and 45 nm and display blue fluorescence with a quantum yield of 8% at excitation/emission wavelengths of 350/445 nm. The fluorescence is pH dependent and decreases under acidic conditions. The doping of the CQDs with selenium and nitrogen was proven by X-ray photoelectron spectroscopy (XPS). Fluorescence is selectively quenched by perfluorooctanoic acid (PFOA), and this is accompanied by a decreased fluorescence lifetime. Quenching is not due to aggregation in view of the unaltered sizes of nanoparticles as revealed by TEM and DLS analyses. UV-vis absorption titration suggested the formation of an excited state complex between SeN-CQDs and PFOA, and quenching originates from the internal electron transfer in the excited state complex. The method was used to detect PFOA quantitatively in the linear range of 10-70 μM with a 1.8 μM detection limit. The nanoprobe has a high selectivity for PFOA over potentially interfering molecules. The practicability of the method was ascertained by accurate detection of PFOA in real samples by the standard addition method. The method may be further improved by tuning the interaction between PFOA and SeN-CQDs through optimizing the doping and the surface composition of the SeN-CQDs. Graphical abstract Schematic presentation of a fluorometric method for perfluorooctanoic acid detection by using a selenium and nitrogen co-doped carbon quantum dots as the fluorescent probe.

Nitrogen and Phosphorus Dual-Doped Multilayer Graphene as Universal Anode for Full Carbon-Based Lithium and Potassium Ion Capacitors

Lithium/potassium ion capacitors (LICs/PICs) have been proposed to bridge the performance gap between high-energy batteries and high-power capacitors. However, their development is hindered by the choice, electrochemical performance, and preparation technique of the battery-type anode materials. Herein, a nitrogen and phosphorus dual-doped multilayer graphene (NPG) material is designed and synthesized through an arc discharge process, using low-cost graphite and solid nitrogen and phosphorus sources. When employed as the anode material, NPG exhibits high capacity, remarkable rate capability, and stable cycling performance in both lithium and potassium ion batteries. This excellent electrochemical performance is ascribed to the synergistic effect of nitrogen and phosphorus doping, which enhances the electrochemical conductivity, provides a higher number of ion storage sites, and leads to increased interlayer spacing. Full carbon-based NPG‖LiPF₆‖active carbon (AC) LICs and NPG‖KPF₆‖AC PICs are assembled and show excellent electrochemical performance, with competitive energy and power densities. This work provides a route for the large-scale production of dual-doped graphene as a universal anode material for high-performance alkali ion batteries and capacitors.