Spin-related Cu-Co pair to increase electrochemical ammonia generation on high-entropy oxides.

The electrochemical conversion of nitrate to ammonia is a way to eliminate nitrate pollutant in water. Cu-Co synergistic effect was found to produce excellent performance in ammonia generation. However, few studies have focused on this effect in high-entropy oxides. Here, we report the spin-related Cu-Co synergistic effect on electrochemical nitrate-to-ammonia conversion using high-entropy oxide Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O. In contrast, the Li-incorporated MgCoNiCuZnO exhibits inferior performance. By correlating the electronic structure, we found that the Co spin states are crucial for the Cu-Co synergistic effect for ammonia generation. The Cu-Co pair with a high spin Co in Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O can facilitate ammonia generation, while a low spin Co in Li-incorporated MgCoNiCuZnO decreases the Cu-Co synergistic effect on ammonia generation. These findings offer important insights in employing the synergistic effect and spin states inside for selective catalysis. It also indicates the generality of the magnetic effect in ammonia synthesis between electrocatalysis and thermal catalysis.

Chloride-Free Electrolyte Based on Tetrabutylammonium Triflate Additive for Extended Anodic Stability in Magnesium Batteries.

Rechargeable magnesium batteries (RMBs) have been proposed as a promising alternative to currently commercialized lithium-ion batteries. However, Mg anode passivation in conventional electrolytes necessitates the use of highly corrosive Cl- ions in the electrolyte. Herein for the first time, we design a chloride-free electrolyte for RMBs with magnesium bis(hexamethyldisilazide) (Mg(HMDS)2) and magnesium triflate (Mg(OTf)2) as the main salts and tetrabutylammonium triflate (TBAOTf) as an additive. The TBAOTf additive improved the dissolution of Mg salts, consequently enhancing the charge-carrying species in the electrolyte. COMSOL studies further revealed desirable Mg growth in our modulated electrolyte, substantiated by homogeneous electric flux distribution across the electrolyte-electrode interface. Post-mortem chemical composition analysis uncovered a MgF2-rich solid electrolyte interphase (SEI) that facilitated exceptional Mg deposition/dissolution reversibility. Our study illustrates a highly promising strategy for synthesizing a corrosion-free and reversible Mg battery electrolyte with a widened anodic stability window of up to 4.43 V.

Structural Design of Electrocatalyst-Decorated MXenes on Sulfur Spheres for Lithium-Sulfur Batteries.

Lithium-sulfur batteries (LSBs) are known to be potential next-generation energy storage devices. Recently, our group reported an LSB cathode made using sulfur spheres that has been spherically templated by MXene nanosheets decorated with CoSe2 nanoparticles, forming a "loose-templating" configuration. It was postulated that the minimal restacking of the outer nanoparticle-decorated MXene layer helps to enable facile ionic transport. However, as the nanosheets do not adhere conformally to the internal sphere's surface, such a configuration can be controversial, thus requiring a more systematic understanding. In this work, we report and quantify for the first time the independent and dependent variables involved in this morphology, allowing us to identify that having smaller nanoparticles resulted in better Li+ ion transport and enhanced electrochemical performances. The optimized cathode structure exhibited an initial specific capacity of 1274 mAh/g and a 0.06% decay rate per cycle at 0.5 C over 1000 cycles in LSBs.

Uncovering the Binder Interactions with S-PAN and MXene for Room Temperature Na-S Batteries.

MXenes and sulfurized polyacrylonitrile (S-PAN) are emerging as possible contenders to resolve the polysulfide dissolution and volumetric expansion issues in sodium-sulfur batteries. Herein, we explore the interactions between Ti3C2Tx MXenes and S-PAN with traditional binders such as polyvinylidene difluoride (PVDF), poly(acrylic acid) (PAA), and carboxymethyl cellulose (CMC) in Na-S batteries for the first time. We hypothesize that the linearity and polarity of the binder significantly influence the dispersion of S-PAN over Ti3C2Tx. The three-dimensional polar CMC binder resulted in better contact surface area with both S-PAN and Ti3C2Tx. Moreover, the improved binding of the discharge products with the CMC binder effectively traps them and prevents unwanted shuttling. Consequently, the Na-S battery using the CMC binder displayed a high initial capacity of 1282 mAh/g(s) at 0.2 C and a low capacity fading of 0.092% per cycle over 300 cycles. This work highlights the importance of understanding MXene-binder interactions in sulfur cathodes.

Metal-Organic-Framework-Derived 3D Hierarchical Matrixes for High-Performance Flexible Li-S Batteries.

Lithium-sulfur (Li-S) batteries have shown exceptional theoretical energy densities, making them a promising candidate for next-generation energy storage systems. However, their practical application is limited by several challenging issues, such as uncontrollable Li dendrite growth, sluggish electrochemical kinetics, and the shuttling effect of lithium polysulfides (LiPSs). To overcome these issues, we designed and synthesized hierarchical matrixes on carbon cloth (CC) by using metal-organic frameworks (MOFs). ZnO nanosheet arrays were used as anode hosts (CC-ZnO) to enable stable Li plating and stripping. The symmetric cell with CC-ZnO@Li was demonstrated to have enhanced cycling stability, with a voltage hysteresis of ∼25 mV for over 800 h at 1 mA cm-2 and 1 mAh cm-2. To address the cathode challenges, we developed a multifunctional CC-NC-Co cathode host with physical confinement, chemical anchoring, and excellent electrocatalysis. The full cells with CC-ZnO@Li anodes and CC-NC-Co@S cathodes exhibited excellent electrochemical performance, with long cycling life (0.02% and 0.03% capacity decay per cycle when cycling 900 times at 0.5 C and 600 times at 1 C, respectively) and outstanding rate performance (793 mAh g-1 at 4 C). Additionally, the pouch cell based on the flexible CC-ZnO@Li anode and CC-NC-Co@S cathode showed good stability in different bending states. Overall, our study presents an effective strategy for preparing flexible Li and S hosts with hierarchical structures derived from MOF, which can pave the way for high-performance Li-S batteries.

Double-Transition-Metal MXene Films Promoting Deeply Rechargeable Magnesium Metal Batteries.

Magnesium metal batteries are promising candidates for next-generation high-energy-density and low-cost energy storage systems. Their application, however, is precluded by infinite relative volume changes and inevitable side reactions of Mg metal anodes. These issues become more pronounced at large areal capacities that are required for practical batteries. Herein, for the first time, double-transition-metal MXene films are developed to promote deeply rechargeable magnesium metal batteries using Mo2 Ti2 C3 as a representative example. The freestanding Mo2 Ti2 C3 films, which are prepared using a simple vacuum filtration method, possess good electronic conductivity, unique surface chemistry, and high mechanical modulus. These superior electro-chemo-mechanical merits of Mo2 Ti2 C3 films help to accelerate electrons/ions transfer, suppress electrolyte decomposition and dead Mg formation, as well as maintain electrode structural integrity during long-term and large-capacity operation. As a result, the as-developed Mo2 Ti2 C3 films exhibit reversible Mg plating/stripping with high Coulombic efficiency of 99.3% at a record-high capacity of 15 mAh cm-2 . This work not only sheds innovative insights into current collector design for deeply cyclable Mg metal anodes, but also paves the way for the application of double-transition-metal MXene materials in other alkali and alkaline earth metal batteries.

A Passivation-Free Solid Electrolyte Interface Regulated by Magnesium Bromide Additive for Highly Reversible Magnesium Batteries.

Highly reversible Mg battery chemistry demands a suitable electrolyte formulation highly compatible with currently available electrodes. In general, conventional electrolytes form a passivation layer on the Mg anode, requiring the use of MgCl2 additives that lead to severe corrosion of cell components and low anodic stability. Herein, for the first time, we conducted a comparative study of a series of Mg halides as potential electrolyte additives in conventional magnesium bis(hexamethyldisilazide)-based electrolytes. A novel electrolyte formulation that includes MgBr2 showed unprecedented performance in magnesium plating/stripping, with an average Coulombic efficiency of 99.26% over 1000 cycles at 0.5 mA/cm2 and 0.5 mAh/cm2. Further analysis revealed the in situ formation of a robust Mg anode-electrolyte interface, which leads to dendrite-free Mg deposition and stable cycling performance in a Mg-Mo6S8 battery over 100 cycles. This study demonstrates the rational formulation of a novel MgBr2-based electrolyte with high anodic stability of 3.1 V for promising future applications.

Additive-Driven Interfacial Engineering of Aluminum Metal Anode for Ultralong Cycling Life.

Rechargeable Al batteries (RAB) are promising candidates for safe and environmentally sustainable battery systems with low-cost investments. However, the currently used aluminum chloride-based electrolytes present a significant challenge to commercialization due to their corrosive nature. Here, we report for the first time, a novel electrolyte combination for RAB based on aluminum trifluoromethanesulfonate (Al(OTf)3) with tetrabutylammonium chloride (TBAC) additive in diglyme. The presence of a mere 0.1 M of TBAC in the Al(OTf)3 electrolyte generates the charge carrying electrochemical species, which forms the basis of reaction at the electrodes. TBAC reduces the charge transfer resistance and the surface activation energy at the anode surface and also augments the dissociation of Al(OTf)3 to generate the solid electrolyte interphase components. Our electrolyte's superiority directly translates into reduced anodic overpotential for cells that ran for 1300 cycles in Al plating/stripping tests, the longest cycling life reported to date. This unique combination of salt and additive is non-corrosive, exhibits a high flash point and is cheaper than traditionally reported RAB electrolyte combinations, which makes it commercially promising. Through this report, we address a major roadblock in the commercialization of RAB and inspire equivalent electrolyte fabrication approaches for other metal anode batteries.

Spherical Templating of CoSe2 Nanoparticle-Decorated MXenes for Lithium-Sulfur Batteries.

Two-dimensional MXenes produce competitive performances when incorporated into lithium-sulfur batteries (LSBs), solving key problems such as the poor electronic conductivity of sulfur and dissolution of its polysulfide intermediates. However, MXene nanosheets are known to easily aggregate and restack during electrode fabrication, filtration, or water removal, limiting their practical applicability. Furthermore, in complex electrocatalytic reactions like the multistep sulfur reduction process in LSBs, MXene alone is insufficient to ensure an optimal reaction pathway. In this work, we demonstrate for the first time a loose templating of sulfur spheres using Ti3C2Tx MXene nanosheets decorated with polymorphic CoSe2 nanoparticles. This work shows that the templating of sulfur spheres using nanoparticle-decorated MXene nanosheets can prevent nanosheet aggregation and exert a strong electrocatalytic effect, thereby enabling improved reaction kinetics and battery performance. The S@MXene-CoSe2 cathode demonstrated a long cycle life of 1000 cycles and a low capacity decay rate of 0.06% per cycle in LSBs.

High Utilization of Composite Magnesium Metal Anodes Enabled by a Magnesiophilic Coating.

Metallic magnesium is a promising high-capacity anode material for energy storage technologies beyond lithium-ion batteries. However, most reported Mg metal anodes are only cyclable under shallow cycling (≤1 mAh cm-2) and thus poor Mg utilization (<3%) conditions, significantly compromising their energy-dense characteristic. Herein, composite Mg metal anodes with high capacity utilization of 75% are achieved by coating magnesiophilic gold nanoparticles on copper foils for the first time. Benefiting from homogeneous ionic flux and uniform deposition morphology, the Mg-plated Au-Cu electrode exhibits high average Coulombic efficiency of 99.16% over 170 h cycling at 75% Mg utilization. Moreover, the full cell based on Mg-plated Au-Cu anode and Mo6S8 cathode achieves superior capacity retention of 80% after 300 cycles at a low negative/positive ratio of 1.33. This work provides a simple yet effective general strategy to enhance Mg utilization and reversibility, which can be extended to other metal anodes as well.