Enhanced catalytic performance of pillared δ-MnO2 with enlarged layer spaces for lithium- and sodium-oxygen batteries: a theoretical investigation

Phosphorous and Nitrogen Dual-Doped Carbon as a Highly Efficient Electrocatalyst for Sodium-Oxygen Batteries

Sodium-oxygen batteries have been regarded as promising energy storage devices due to their low overpotential and high energy density. Its applications, however, still face formidable challenges due to the lack of understanding about the influence of electrocatalysts on the discharge products. Here, a phosphorous and nitrogen dual-doped carbon (PNDC) based cathode is synthesized to increase the electrocatalytic activity and to stabilize the NaO2 superoxide nanoparticle discharge products, leading to enhanced cycling stability when compared to the nitrogen-doped carbon (NDC). The PNDC air cathode exhibits a low overpotential (0.36 V) and long cycling stability (120 cycles). The reversible formation/decomposition and stabilization of the NaO2 discharge products are clearly proven by in-situ synchrotron X-ray diffraction and ex-situ X-ray diffraction. Based on the density functional theory calculation, the PNDC has much stronger adsorption (-2.85 eV) for NaO2 than that of NDC (-1.80 eV), which could efficiently stabilize the NaO2 discharge products.

Fast carrier diffusion via synergistic effects between lithium-ions and polarons in rutile TiO2

Carrier mobility in titanium dioxide (TiO₂) systems is a key factor for their application as energy materials, especially in solar cells and lithium-ion batteries. Studies on the diffusion of Li-ions and polarons in rutile TiO₂ systems have attracted extensive attention. However, how their interaction affects the diffusion of Li-ions and electron polarons is largely unclear and related studies are relatively lacking. By using first-principles calculations, we systematically investigate the interaction between the intercalated Li-ions and electron polarons in rutile TiO₂ materials. Our analysis shows that the diffusion barrier of the electron polarons decreases around the Li-ion. The interaction between the Li-ions and polarons would benefit their synergistic diffusion both in the pristine and defective rutile TiO₂ systems. Our study reveals the synergistic effects between the ions and polarons, which is important for understanding the carrier properties in TiO₂ systems and in further improving the performance of energy materials.

Electrons and phonons of the discharge products in the lithium-oxygen and lithium-sulfur batteries from first-principles calculations

Batteries have become a ubiquitous daily necessity, which are popularly applied to mobile phones and electric vehicles according to their size. Improving the battery cycle life and storage is important, but unexpected discharge products still restrict the upper limit of batter performance such as Li₂O₂, LiO₂, and Li₂S. In this study, we calculated electrons and phonons presenting the basic energy states in crystal using the first-principles calculations. The Li₂O₂ and Li₂S are almost insulating due to the wide bandgap from their electronic structure, and doped-active p-orbital may be one of the pathways to improve crystal conduction due to the tendency of the density of states. The LiO₂ is metallic, and the electronic structure and phonons show that the discharge products have an ionic feature. In addition, the ionic crystal can produce a larger DC permittivity because it possesses macroscopic polarisation. For Li₂O₂ and Li₂S, the Raman peak of the O-O bonding is strong, while the Raman peak of the S-ion is very weak. The enhanced Raman peak of the S-ion presents a possibility to prevent the shuttle effect in Li-S batteries.

A new high capacity cathode material for Li/Na-ion batteries: dihafnium sulfide (Hf2S)

Structural and electronic properties of the newly synthesized dihafnium sulfide (Hf₂S) monolayer were investigated in this study. The Hf₂S monolayer is a magnetic metal and it retains its metallicity despite external factors, such as tensile/compressive strain or various atom terminations. This robust metallic Hf₂S monolayer has a high storage capacity of 1377.7 mA h g^-1 for both Li and Na atoms, which is much higher than conventional battery electrodes. The minimum diffusion barrier value is 131 meV for Li, and 117 meV for Na. The average open-circuit voltage values of Li and Na were calculated as 2.37 V and 1.69 V, respectively. Investigation of the mechanical properties showed that it is softer than graphene due to the Young's modulus of 111.7 N m^-1, but comparable with the molybdenum disulfide (MoS₂) monolayer. In light of the results of this study, the Hf₂S monolayer can serve as a useful cathode material for Li-ion and Na-ion batteries. In addition, the interactions of the Hf₂S monolayer with aluminium nitride (AlN) and MoS₂ monolayers were presented. While AlN behaves like a substrate for the Hf₂S monolayer, the MoS₂ monolayer forms a heterostructure with the Hf₂S monolayer. This study is a guide for the Hf₂S monolayer and its functions in nanotechnology.

Investigating the role of structural water on the electrochemical properties of α-V2O5 through density functional theory

The α polymorph of V₂O₅ is one of the few known cathodes capable of reversibly intercalating multivalent ions such as Mg, Ca, Zn and Al, but suffers from sluggish diffusion kinetics. The role of H₂O within the electrolyte and between the layers of the structure in the form of a xerogel/aerogel structure, though, has been shown to lower diffusion barriers and lead to other improved electrochemical properties. This density functional theory study systematically investigates how and why the presence of structural H₂O within α-V₂O₅ changes the resulting structure, voltage, and diffusion kinetics for the intercalation of Li, Na, Mg, Ca, Zn, and Al. We found that the coordination of H₂O molecules with the ion leads to an improvement in voltage and energy density for all ions. This voltage increase was attributed to the extra host sites for electrons present with H₂O, thus leading to a stronger ionization of the ion and a higher voltage. We also found that the increase in interlayer distance and a potential "charge shielding" effect drastically changes the electrostatic environment and the resulting diffusion kinetics. For Mg and Ca, this resulted in a decrease in diffusion barrier from 1.3 eV and 2.0 eV to 0.89 eV and 0.4 eV, respectively. We hope that our study motivates similar research regarding the role of water in both V₂O₅ xerogels/aerogels and other layered transition metal oxides.

Tuning the structural stability and electrochemical properties in graphene anode materials by B doping: a first-principles study

The first-principles method of density functional theory (DFT) is used to study the structural stability and electrochemical properties of B doped graphene with concentrations of 3.125%, 6.25% and 18.75% respectively, and their lithium storage mechanism and characteristics are further studied. The results show that the doped systems all have negative adsorption energy, indicating that the structures can exist stably, and the adsorption energy of lithium ions on graphene decreases with the increase of B doping concentration. Among them, the B₆C₂₆ structure has the lowest adsorption energy and can adsorb more lithium ions. The density of states indicates that doping with B can increase the conductivity of graphene greatly. Subsequently, the CI-NEB method to search for the transition state of the doped structure is used, showing that the B₆C₂₆ structure has the lowest diffusion barrier and good rate performance. Therefore, these findings provide a certain research foundation for the development and application of lithium-ion battery anode materials.

Theoretical Investigation of a Tetrazine Based Covalent Organic Framework as a Promising Anode Material for Sodium/Calcium Ion Batteries

Nowadays, a great attention is being directed towards the development of promising elec- trode materials for non-lithium rechargeable batteries such as sodium and calcium ion batter- ies (SIBs and CIBs),...