Singlet oxygen vs triplet oxygen: Functions of 2D-MoO3 catalyst in conquering catastrophic parasitic-reactions in lithium- and sodium-oxygen batteries

Aprotic electrolyte alkali metal-oxygen batteries that possess a higher energy density than lithium-ion batteries do stand out to be the most promising next-generation environmental-friendly energy source. Parasitic reactions caused by...

Pulse Discharging of Sodium-Oxygen Batteries to Enhance Cathode Utilization

Using sodium metal in sodium-oxygen batteries with aprotic electrolyte enables achieving a very high theoretical energy density. However, the promised values for energy density and capacity are not met in practical studies yet due to poor utilization of the void space in the cathode during battery discharge. In this work, we achieve better cathode utilization and higher discharge capacities by using pulse discharging. We optimize the chosen resting-to-pulse times, the applied current density, and elucidate that three-dimensional cathode materials yield higher capacities compared to two-dimensional ones. By implication, the pulse discharging mode ensures better supply with dissolved oxygen within the cathode. The higher amount of dissolved oxygen accumulated during the resting period after a current pulse is essential to form more of the discharge product, i.e., the metal oxide sodium superoxide. Interestingly, we show for the first time that the superoxide is deposited in a very unusual form of stacked and highly oriented crystal layers. Our findings on the pulse discharging can be transferred to other metal-oxygen battery systems and might assist in achieving their full potential regarding practical energy density.

Ultra-small MnCo2O4 nanocrystals decorated on nitrogen-enriched carbon nanofibers as oxygen cathode for Li-O2 batteries

Ultra-small MnCo2O4 nanocrystals/nitrogen enriched carbon nanofiber composites, with particle size as small as 2–4[Formula: see text]nm and nitrogen content as high as [Formula: see text][Formula: see text]at.%, were designed and used as oxygen cathode materials for Li-O2 batteries, with an aprotic electrolyte. Via an in-situ nucleation and growth, the morphology, size and distribution of MnCo2O4 nanocrystals were well controlled on surface of nanofibers, with strong interfacial interactions between the two components. Benefitting from the unique microstructure and high-level nitrogen doping, the MnCo2O4/NCF composite can deliver discharge and charge capacities of 4147.8 and 3842.8[Formula: see text]mAh/g as oxygen cathode materials, and columbic efficiencies are about 92.6%. More importantly, the discharge products can completely decompose in charging process as evidenced by an ex-situ FESEM investigation, while for pure NCF cathode, particle and plate-like Li2O2 were still observed on its surface, which confirmed that the MnCo2O4/NCF composite has a superior electrocatalytic activity that that of NCF.

The Role of Interface in Stabilizing Reaction Intermediates for Hydrogen Evolution in Aprotic Li-Ion Battery Electrolyte

p { margin-bottom: 0.1in; direction: ltr; color: rgb(0, 0, 10); line-height: 120%; text-align: left; }p.western { font-size: 12pt; }p.cjk { font-size: 12pt; }a:link { color: rgb(5, 99, 193); } <p> By combining idealized experiments with realistic quantum mechanical simulations of the interface, we investigate electro-reduction reactions of HF and water impurities on the single crystal (111) facets of Au, Pt, Ir and Cu in an organic aprotic electrolyte, 1M LiPF₆ in EC/EMC 3:7w (LP57), which are common reactions happening during the formation of the SEI on graphite. In our previous work, we have established that the LiF formation, accompanied with H₂ evolution, is caused by a reduction of HF impurities and requires the presence of Li at the interface, which catalyzes the HF dissociation. In the present paper, we find that the measured potential of the electrochemical response for these reduction reactions correlates with the work function of the electrode surfaces and that the work function determines the potential for Li⁺ adsorption. The reaction path is investigated further by electrochemical simulations suggesting that the overpotential of the reaction is related to stabilizing the active structure of the interface having Li⁺ adsorbed. The Li⁺ is needed to facilitate the dissociation of HF which is the source of proton. Further experiments on the other proton sources, water and methanesulfonic acid, show that if the hydrogen evolution involves negatively charged intermediates, F^- or HO^-, a cation at the interface can stabilize them and facilitate the reaction kinetics. When the proton source is already significantly dissociated (in the case of a strong acid), there is no negatively charged intermediate and thus the hydrogen evolution can proceed at much lower overpotentials. This reveals a situation where the overpotential for electrocatalysis is related to stabilizing the active structure of the interface, facilitating the reaction rather than providing the reaction energy. This has implications for the SEI layer formation in Li-ion batteries and for reduction reactions in alkaline environment as well as for design principles for better electrodes.</p>