Low Migration Energy Barrier Research Articles

Determination of Dynamically Stable Electrenes toward Ultrafast Charging Battery Applications

Electrenes, an atomically thin form of layered electrides, are very recent members of the 2D materials family. In this work, we employed first-principle calculations to determine stable, exfoliatable, and application-promising 2D electrene materials among possible M2X compounds, where M is a group II-A metal and X is a nonmetal element (C, N, P, As, and Sb). The promise of stable electrene compounds for battery applications is assessed via their exfoliation energy, adsorption properties, and migration energy barriers toward relevant Li, Na, K, and Ca atoms. Our calculations revealed five new stable electrene candidates in addition to previously known Ca2N and Sr2N. Among these seven dynamically stable electrenes, Ba2As, Ba2P, Ba2Sb, Ca2N, Sr2N, and Sr2P are found to be very promising for either K or Na ion batteries due to their extremely low migration energy barriers (5-16 meV), which roughly demonstrates 105 times higher mobility than graphene and two to four times higher mobility than other promising 2D materials such as MXene (Mo2C).

Promoting lithium polysulfide/sulfide redox kinetics by the catalyzing of zinc sulfide for high performance lithium-sulfur battery

Entrapping polysulfide from dissolution into electrolyte by strong chemisorption of polar materials has been widely reported in lithium-sulfur (Li-S) battery. Here, for the first time, zinc sulfide (ZnS) was demonstrated as an activation catalyst in Li-S battery to suppress the soluble polysulfide shuttle effect by powering kinetics redox reactions of lithium polysulfide/sulfide. Kinetic analyses comprehensively identify that ZnS not only facilities polysulfide redox kinetics in liquid phase (Li2S8→Li2S6→Li2S4), but also promotes the effective decompositions of lithium sulfide (Li2S). Furthermore, first-principle calculations confirm that the low lithium ion diffusion barrier on the surface of ZnS promotes the redox reaction between lithium ion and sulfur species; and the low migration energy barrier of polysulfide on its surface guarantees the fast diffussion of polysulfides from the ZnS surface to the nearby conductive substrate, thus effectively smoothes polysulfides’ entrapping-diffusion-conversion mechanism across the ZnS interface, resulting in the highly reversible electrochemical performance. As evidenced by the ex situ SEM and visible experiment, the reaction between migrated sulfur species and lithium anode was significantly alleviated, and the insulating Li2S/Li2S2 was uniformly deposited on the ZnS-CB/S cathode. This ZnS cathode based Li-S battery exhibits outstanding performance including an excellent retained discharge specific capacity of 589 mA h gsul−1 with the high sulfur loading of 7 mg cm−2 (200 cycles) and extended cycling stability at the high current rate of 2 C, 5 C (632, 388 mA h gsul−1 after 1000 cycles).

Reinvestigation of Na2Fe2(C2O4)3·2H2O: An Iron-Based Positive Electrode for Secondary Batteries

We have reinvestigated the polyanionic compound Na2Fe2(C2O4)3·2H2O, previously reported to be electrochemically inactive in lithium-ion batteries (LIBs), as a positive electrode for sodium-ion batteries (NIBs). The present study demonstrates that it is capable of delivering a reversible capacity close to its theoretical value (117 mA h g–1) with three redox plateaus at 2.9, 3.3, and 3.6 V versus Na/Na+ in the potential range 1.7–4.2 V. The obtained energy density of 326 W h kg–1 is among the highest of all reported polyanionic cathodes in NIBs. The origin of the electrochemical activity can be traced back to the electronic structure of the compound and the low migration energy barrier of the alkali ion observed in first-principles density-functional theory calculations.

First principles prediction of two-dimensional tungsten carbide (W2C) monolayer and its Li storage capability

First-principles calculations are performed to study the electronic properties and Li ion storage capabilities of the proposed two-dimensional (2D) W2C monolayer. After ensuring its dynamical and thermal stabilities, we found that the Li atom is strongly bound to the W2C monolayer. The metallic character of both pristine and doped W2C ensures a good electronic conduction that is essential for an optimal anode material. Low migration energy barrier is predicted as small as 45 meV for Li, which results in the very fast diffusion of Li atom on W2C. We found the storage capacity is 259 mA h g−1. The calculated average voltage of 0.55 V for Li makes W2C attractive for low charging voltage applications.

Fast and Huge Anisotropic Diffusion of Cu (Ag) and Its Resistance on the Sn Self-diffusivity in Solid β–Sn

The site preferences, fast and huge anisotropic diffusion mechanisms of Cu (Ag) in β–Sn as well as their reduction on the self-diffusivity of Sn, have been investigated using the first-principles and ab initio molecular dynamics methods. We have found that Cu prefers the interstitial site, whereas Ag is preferable in the substitutional site, which is mostly dominated by their different size factors. Electronic structure further evidences that the d–s hybridization between the solute and the host atom also contributes to the site preferences. It is also deduced that the fast diffusion of Cu (Ag) is mostly due to the interstitial diffusion mechanism and their diffusivity can be correlated with the amount of their respective interstitial solution. Their faster diffusion along the c-axis can be attributed to the extremely low migration energy barrier caused by the straight tunnel of considerable size with the screw axis symmetry of 2π/4 along the c-axis. Furthermore, it is found that during the process of diffusion the interstitially dissolved Cu (Ag) atoms would combine with the nearby Sn-vacancy and further annihilate the vacancy, thereby reducing the self-diffusion of Sn.

Diffusion of small He clusters in bulk and grain boundaries in α-Fe

The diffusion properties of He interstitials and He clusters in the bulk and grain boundaries (GBs) of α-Fe have been studied using molecular dynamics with a newly developed Fe–He potential. The low migration energy barrier for a single He interstitial in the bulk is consistent with that obtained using ab initio methods. Small He clusters can migrate at low temperatures, but at higher temperatures they will kick out a self-interstitial atom (SIA) and become trapped by the vacancy, forming an He-vacancy complex. It is of great interest to note that small Hen-vacancy clusters (n<5) in the bulk are able to absorb an SIA, and the clusters become mobile again. Trapping and de-trapping of He clusters by emitting and absorbing an SIA represent an important dynamic process that provides a mechanism for the diffusion of He clusters and the nucleation of He bubbles in bulk Fe, particularly under irradiation in which numerous SIAs and vacancies are constantly being produced. A single He interstitial can migrate one-dimensionally or two-dimensionally within GBs, depending on the GB structure. Small interstitial Hen clusters (n∼1–10) can easily kick out an SIA, and become trapped by the vacancy, while the SIA quickly diffuses away from the clusters, disappearing into the GB, such that de-trapping of the He clusters by absorbing an SIA is less likely to occur. This suggests that small He clusters may be treated as relatively immobile defects in GBs. The different behavior of He clusters in the bulk compared to their behavior in GBs may explain the different He bubble sizes experimentally observed in the bulk and in GBs in reduced activation ferritic/martensitic steels that have been simultaneously neutron irradiated and He implanted.

Mixed ionic/electronic conductors Sr2Fe2O5 and Sr4Fe6O13: atomic-scale studies of defects and ion migration

Atomistic simulations of the structures and defect energetics of two mixed-conducting strontium ferrite materials, Sr2Fe2O5 and Sr4Fe6O13, are reported. Oxygen Frenkel defects are found to be the predominant intrinsic defects in both materials, with the Frenkel energy in the intergrowth structure, Sr4Fe6O13, being much less than that in Sr2Fe2O5 or other known oxide ion conductors. Formation of electronic defects under oxidizing and reducing conditions is calculated to be more favourable in the Sr4Fe6O13 intergrowth structure than in Sr2Fe2O5. Comparison of solution energies for cobalt incorporation shows Sr4Fe6O13 has a slight preference for Co2+ being located on lower coordination sites, whereas in Sr2Fe2O5, Co2+ ions located on tetrahedral sites are most favourable. Binding energy calculations suggest the possible formation of Co2+–vacancy clusters with increasing Co2+ concentration. The rapid oxide ion conductivity in Sr4Fe6−xCoxO13±δ membranes is thought to arise from a combination of factors: a low oxygen Frenkel energy (to produce an intrinsic population of mobile interstitial oxide ions and vacancies), low migration energy barriers, and ease of distortion of polyhedra.