Unveiling the Electrocatalytic Activity of 1T'-MoSe2 on Lithium-Polysulfide Conversion Reactions.

The dissolution of intermediate lithium polysulfides (LiPS) into an electrolyte and their shuttling between the electrodes have been the primary bottlenecks for the commercialization of high-energy density lithium-sulfur (Li-S) batteries. While several two-dimensional (2D) materials have been deployed in recent years to mitigate these issues, their activity is strictly restricted to their edge-plane-based active sites. Herein, for the first time, we have explored a phase transformation phenomenon in a 2D material to enhance the number of active sites and electrocatalytic activity toward LiPS redox reactions. Detailed theoretical calculations demonstrate that phase transformation from the 2H to 1T' phase in a MoSe2 material activates the basal planes that allow for LiPS adsorption. The corresponding transformation mechanism and LiPS adsorption capabilities of the as-formed 1T'-MoSe2 were elucidated experimentally using microscopic and spectroscopic techniques. Further, the electrochemical evaluation of phase-transformed MoSe2 revealed its strong electrocatalytic activity toward LiPS reduction and their oxidation reactions. The 1T'-MoSe2-based cathode hosts for sulfur later provide a superior cycling performance of over 250 cycles with a capacity loss of only 0.15% per cycle along with an excellent Coulombic efficiency of 99.6%.

Light-Assisted Rechargeable Lithium Batteries: Organic Molecules for Simultaneous Energy Harvesting and Storage.

Lithium batteries that could be charged on exposure to sunlight will bring exciting new energy storage technologies. Here, we report a photorechargeable lithium battery employing nature-derived organic molecules as a photoactive and lithium storage electrode material. By absorbing sunlight of a desired frequency, lithiated tetrakislawsone electrodes generate electron-hole pairs. The holes oxidize the lithiated tetrakislawsone to tetrakislawsone while the generated electrons flow from the tetrakislawsone cathode to the Li metal anode. During electrochemical operation, the observed rise in charging current, specific capacity, and Coulombic efficiency under light irradiation in contrast to the absence of light indicates that the quinone-based organic electrode is acting as both photoactive and lithium storage material. Careful selection of electrode materials with optimal bandgap to absorb the intended frequency of sunlight and functional groups to accept Li-ions reversibly is a key to the progress of solar rechargeable batteries.

Atomically Engineered Transition Metal Dichalcogenides for Liquid Polysulfide Adsorption and Their Effective Conversion in Li-S Batteries.

Curtailing the polysulfide shuttle by anchoring the intermediate lithium polysulfides (LiPS) within the electrode structure is essential to impede the rapid capacity fade in lithium-sulfur (Li-S) batteries. While most of the contemporary Li-S cathode surfaces are capable of entrapping certain LiPS, developing a unique electrode material that can adsorb all the intermediates of sulfur redox is imperative. Herein, we report doping of the MoS2 atomic structure with nickel (Ni@1TMoS2) to modulate its absorption capability toward all LiPS and function as an electrocatalyst for Li-S redox. Detailed in situ and ex situ spectroscopic analysis revealed that both Ni and Mo sites chemically anchor all the intermediate of LiPS. Electrochemical studies and detailed kinetics analysis suggested that the conversion of liquid LiPS to solid end products are facilitated on the Ni@1TMoS2 electrocatalytic surface. Further, the employment of the Ni@1TMoS2 electrocatalyst enhances the Li+ diffusion coefficient, thus contributing to the realization of a high capacity of 1107 mA h g-1 at 0.2C with a very limited capacity fade of 0.19% per cycle for over 100 cycles. In addition, this cathode demonstrated an excellent high rate and long cycling performance for over 300 cycles at a 1C rate.

Bioderived Molecular Electrodes for Next-Generation Energy-Storage Materials.

Invited for this month's cover are the groups of George John at the City College of New York-CUNY, Leela R. Arava at Wayne State University, and Pulickel Ajayan at Rice University. The image portraits future prospects of bioderived molecular electrodes for next-generation energy-storage materials. The Minireview itself is available at 10.1002/cssc.201903589.

Bioderived Molecular Electrodes for Next-Generation Energy-Storage Materials.

Nature-derived organic small molecules, as energy-storage materials, provide low-cost, recyclable, and non-toxic alternatives to inorganic and polymer electrodes for lithium-/sodium-ion batteries and beyond. Some organic carbonyl compounds have met or exceeded the voltages and gravimetric storage capacities achieved by traditional transition metal oxide-based compounds due to the metal-ion coupled redox and facile electron-transport capability of functional groups. Stability issues that previously limited the capacity of small organic molecules can be remediated with reactions to form insoluble salts, noncovalent interactions (hydrogen bonding and π stacking), loading onto substrates, and careful electrolyte selection. The cost-effectiveness and sustainability of organic materials may further be improved by employing porphyrin-based electrodes and multivalent-ion batteries utilizing abundant metals, such as aluminum and zinc. Finally, redox flow batteries take advantage of the solubility of organics for the development of scalable, high power density, and safe energy-storage devices based on aqueous electrolytes. Herein, the advantages and prospects of small molecule-based electrodes, with a focus on nature-derived organic and biomimetic materials, to realize the next-generation of green battery chemistry are reviewed.

In Situ Electrochemical Mapping of Lithium-Sulfur Battery Interfaces Using AFM-SECM.

Although lithium-sulfur (Li-S) batteries are explored extensively, several features of the lithium polysulfides (LiPS) redox mechanism at the electrode/electrolyte interface still remain unclear. Though various in situ and ex situ characterization techniques have been deployed in recent years, many spatial aspects related to the local electrochemical phenomena of the Li-S electrode are not elucidated. Herein, we introduce the atomic-force-microscopy-based scanning electrochemical microscopy (AFM-SECM) technique to study the Li-S interfacial redox reactions at nanoscale spatial resolution in real time. In situ electrochemical and alternating current (AC) phase mappings of Li2S particle during oxidation directly distinguished the presence of both conducting and insulating regions within itself. During charging, the conducting part undergoes dissolution, whereas the insulating part, predominantly Li2S, chemically/electrochemically reacts with intermediate LiPS. At higher oxidation potentials, as-reacted LiPS turns into insulating products, which accumulate over cycling, resulting in reduction of active material utilization and ultimately leading to capacity fade. The interdependence of the topography and electrochemical oxidative behavior of Li2S on the carbon surface by AFM-SECM reveals the Li2S morphology-activity relationship and provides new insights into the capacity fading mechanism in Li-S batteries.