Highly Reversible Room-Temperature Sulfur/Long-Chain Sodium Polysulfide Batteries

In a room-temperature sodium–sulfur (RT Na–S) battery, the complicated reduction reaction of the sulfur cathode generally involves two main steps: (i) transformation of elemental sulfur into long-chain soluble sodium polysulfides (Na₂S_<i>n</i> 4 ≤ <i>n</i> ≤ 8) and (ii) conversion of the long-chain sodium polysulfides into solid-state short-chain polysulfide Na₂S₂ or disulfide Na₂S. It is found that the slow kinetics of the second step limits the efficiency of discharge and induces irreversible capacity loss during cycling. Accordingly, we present here a RT Na–S cell operated with the sulfur/long-chain sodium polysulfide redox couple to avoid the capacity fade. An advanced cathode structure has been developed by inserting a carbon nanofoam interlayer between the sulfur cathode and the separator to localize the soluble polysulfide species and prevent its migration to the anode. The highly reversible sulfur/long-chain sodium polysulfide cell presented here can provide a stable output energy density of 450 Wh kg^–1 at an extremely low energy cost of ∼$10 kWh^–1 (based on the active material of anode and cathode)

Electrochemical Energy Storage with an Aqueous Quinone–Air Chemistry

Organic electrode materials such as quinones are drawing rising attention as promising redox-active materials for the development of rechargeable batteries. In aqueous solutions, the redox potential of quinones is dependent on the alkalinity and acidity of the medium. Under an alkaline condition, the oxidation potential of hydroquinone (existing as diphenolate) is ca. 0.8 V lower than that under an acidic condition. On the other hand, under an acidic condition, the reduction potential of oxygen is ca. 0.8 V higher than that under an alkaline condition. By taking these advantages, a quinone–air cell with a rational voltage is strategically demonstrated with an alkaline anode electrolyte and an acidic cathode electrolyte, which are physically separated by a Na⁺-ion conductive solid-state electrolyte membrane. The Na⁺-ions shuttling through the solid-state membrane act as ionic mediators/messengers to sustain and link the redox reactions at the two electrodes

Application of Derivative Voltammetry in the Analysis of Methanol Oxidation Reaction

Application of derivative voltammetry to methanol oxidation reaction (MOR) has been studied, and its advantage in the analysis of various aspects of MOR with regard to direct methanol fuel cell has been delineated. The derivative technique of voltammetric analysis was employed in evaluating and comparing MOR activities of carbon-supported mono-, bi-, and tri-metallic electrocatalysts. Significant enhancement in the accuracy of estimating voltammetric peak potential and onset potential of methanol oxidation current can be achieved in derivative voltammetry. Furthermore, the better signal-to-noise ratio of derivative voltammetry practically eliminates charging current. From a mechanistic point of view, derivative voltammetry is highly sensitive in resolving a peak due to parallel path mechanism. Electrochemical stability of anode catalysts can be better evaluated and monitored employing derivative voltammetry. Nevertheless, the derivative technique is simple and does not require further instrumentation or cell assembly

Performance Enhancement and Mechanistic Studies of Room-Temperature Sodium–Sulfur Batteries with a Carbon-Coated Functional Nafion Separator and a Na₂S/Activated Carbon Nanofiber Cathode

Operation of sodium–sulfur batteries at room temperature has been proposed and studied for about a decade, but polysulfide-shuttle through the traditional battery separator and low-utilization of the sulfur cathode commonly have been the major challenges. Also, because of the highly active nature of the sodium metal, the conventional room temperature sodium–sulfur (RT Na–S) battery concept with the sodium–metal anode and elemental sulfur cathode imposes serious safety concerns. To overcome the above difficulties, we present here a RT Na–S system with an advanced membrane-electrode-assembly (MEA) comprising a carbon-coated, presodiated Nafion membrane (Na-Nafion) and a sodium sulfide (Na₂S) cathode. The Na-Nafion membrane provides a facile Na⁺-ion conductive path and serves as a cation-selective shield to prevent the migration of the polysulfides to the anode. The carbon coating on the Na-Nafion plays an upper-current-collector role and thereby improves the electrochemical utilization of the active Na₂S. Employing Na₂S as the cathode provides a pathway to develop the RT Na–S batteries with sodium–metal-free anodes. The RT Na–S battery with the above MEA exhibits remarkably enhanced capacity and cyclability in contrast to the Na–S batteries with the conventional electrolyte–separator configuration. Mechanistic studies reveal that the suppression of polysulfide migration through the Na-Nafion is due to size and electronic effects

Microwave-Assisted Solvothermal Synthesis of Spinel AV₂O₄ (M = Mg, Mn, Fe, and Co)

Lower-valent vanadium oxide spinels AV₂O₄ (A = Mg, Mn, Fe, and Co) consisting of A²⁺ and V³⁺ ions have been synthesized by a low-temperature microwave-assisted solvothermal (MW-ST) synthesis process in a tetraethylene glycol (TEG) medium. The oxides are formed within a short reaction time of 30 min at 300 °C. Subsequent postheat treatment of the oxides at elevated temperatures in inert or reducing atmospheres results in an instability of the spinel phase, especially CoV₂O₄ due to the ease of formation of metallic Co, demonstrating the advantage of the low-temperature MW-ST process in accessing these oxides. This MW-ST synthesis approach is attractive for synthesizing other lower-valent transition-metal oxides that are otherwise difficult to obtain by conventional synthesis methods and for subsequent study of their unique physical and chemical properties

Effects of Chemical versus Electrochemical Delithiation on the Oxygen Evolution Reaction Activity of Nickel-Rich Layered Li<i>M</i>O₂

Nickel-rich layered Li<i>M</i>O₂ (<i>M</i> = transition metal) oxides doped with iron exhibit high oxygen evolution reaction (OER) activity in alkaline electrolytes. The Li<i>M</i>O₂ oxides offer the possibility of investigating the influence of the number of d electrons on OER by tuning the oxidation state of <i>M</i> via chemical or electrochemical delithiation. Accordingly, we investigate here the electrocatalytic behavior of LiNi_0.7Co_0.3O₂ and LiNi_0.7Co_0.2Fe_0.1O₂ before and after chemical delithiation. In addition to varying the oxidation state of the transition-metal ions, we find that chemical delithiation also affects the local chemical environment and morphology. The electrochemical response differs depending on whether the delithiation occurred ex situ chemically or in situ during the electrocatalysis. The results point to the important role of in situ transformation in Li<i>M</i>O₂ in alkaline electrolytes during electrocatalytic cycling

Bi_0.94Sb_1.06S₃ Nanorod Cluster Anodes for Sodium-Ion Batteries: Enhanced Reversibility by the Synergistic Effect of the Bi₂S₃–Sb₂S₃ Solid Solution

Bi_0.94Sb_1.06S₃ solid solution anode with a nanorod cluster morphology has been synthesized by a hydrothermal reaction and investigated in sodium-ion batteries in comparison to the binary counterparts Bi₂S₃ and Sb₂S₃. The Bi_0.94Sb_1.06S₃–graphite composite obtained by a mechanical milling of 80 wt % Bi_0.94Sb_1.06S₃ and 20 wt % graphite exhibits much improved cycle stability with 79% capacity retention after 200 cycles in the full voltage window of 0.01–2.8 V compared to its Bi₂S₃–graphite (58% retention) and Sb₂S₃–graphite (10% retention) counterparts, demonstrating a synergistic effect. Cyclic voltammetry scans indicate that the polarization overpotential associated with the conversion reaction in Bi_0.94Sb_1.06S₃ is lower than those in Sb₂S₃ and Bi₂S₃. Cycling under controlled voltage windows of 2.8–0.85 V corresponding to a conversion reaction and 0.85–0.01 V corresponding to an alloying reaction reveals both highly reversible conversion and alloying reactions in the Bi_0.94Sb_1.06S₃ solid solution anode compared to that in the binary counterpart anodes

Performance Enhancement and Mechanistic Studies of Magnesium–Sulfur Cells with an Advanced Cathode Structure

Magnesium–sulfur cells based on abundant, safe Mg and S are demonstrated with a cathode containing a preactivated carbon nanofiber (CNF) electrode matrix filled with sulfur active material and a CNF-coated separator. The CNF coating on the separator serves as a polysulfide trapper and an upper current collector for facilitating high sulfur utilization and enhancing the cycle life. Mechanisms regarding the performance enhancement and the charge–discharge processes of the Mg–S cells are investigated with spectroscopic, microscopic, and electrochemical analyses

Electrochemical Energy Storage with an Aqueous Zinc–Quinone Chemistry Enabled by a Mediator-Ion Solid Electrolyte

Quinone series of organics are promising electrode materials for the development of low-cost, sustainable, environmentally benign electrochemical energy storage technologies. However, the redox potential of quinones is sensitively dependent on the acidity and alkalinity of aqueous solutions. This study demonstrates a high-voltage aqueous zinc–quinone battery with an alkaline anode electrolyte (anolyte) and an acidic cathode electrolyte (catholyte), which are separated by a sodium mediator-ion solid electrolyte membrane. The redox chemistries of the acidic quinone cathode and the alkaline Zn anode are ionically linked by the shuttling of sodium mediator ions through the solid electrolyte