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Register a new userGraphyne as the anode material of magnesium-ion batteries: ab initio study
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Added by
Ismaeil Abdolhosseini Sarsari
Publication date
2018
fields
Physics
and research's language is
English
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Graphyne, a single atomic layer structure of the carbon six-member rings connected by one acetilenic linkage, is a promising anode of rechargeable batteries. In this paper, a first-principle study has been carried out on graphyne as a new candidate for the anode material of magnesium-ion batteries, using density functional theory calculations. The main focus is on the magnesium adsorption on graphyne surface. The structural properties such as adsorption height and energy, the most stable adsorption sites, the Band structure and DOS of the pristine graphyne the diverse Mg-decorated graphyne structures, and energy barrier against Mg diffusion are also calculated. As a consequence of the band structure and DOS of graphyne structures, it is found that the pristine graphyne and the Mg-decorated graphyne structures show a semiconducting nature and metallic behavior, respectively. Moreover, the migration behavior of Mg on graphyne for the main diffusion paths is determined by the Nudged Elastic Band (NEB) method.
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Magnesium alanate Mg(AlH4)2 has recently raised interest as a potential material for hydrogen storage. We apply ab initio calculations to characterize structural, electronic and energetic properties of Mg(AlH4)2. Density functional theory calculations within the generalized gradient approximation (GGA) are used to optimize the geometry and obtain the electronic structure. The latter is also studied by quasi-particle calculations at the GW level. Mg(AlH4)2 is a large band gap insulator with a fundamental band gap of 6.5 eV. The hydrogen atoms are bonded in AlH4 complexes, whose states dominate both the valence and the conduction bands. On the basis of total energies, the formation enthalpy of Mg(AlH4)2 with respect to bulk magnesium, bulk aluminum and hydrogen gas is 0.17 eV/H2 (at T = 0). Including corrections due to the zero point vibrations of the hydrogen atoms this number decreases to 0.10 eV/H2. The enthalpy of the dehydrogenation reaction Mg(AlH4)2 -> MgH2 +2Al+3H2(g) is close to zero, which impairs the potential usefulness of magnesium alanate as a hydrogen storage material.
We present a rigorous analysis of the Magnesium Aluminum Chloro Complex (MACC) in tetrahydrofuran (THF), one of the few electrolytes that can reversibly plate and strip Mg. We use emph{ab initio} calculations and classical molecular dynamics simulations to interrogate the MACC electrolyte composition with the goal of addressing two urgent questions that have puzzled battery researchers: emph{i}) the functional species of the electrolyte, and emph{ii}) the complex equilibria regulating the MACC speciation after prolonged electrochemical cycling, a process termed as conditioning, and after prolonged inactivity, a process called aging. A general computational strategy to untangle the complex structure of electrolytes, ionic liquids and other liquid media is presented. The analysis of formation energies and grand-potential phase diagrams of Mg-Al-Cl-THF suggests that the MACC electrolyte bears a simple chemical structure with few simple constituents, namely the electro-active species MgCl$^+$ and AlCl$_4^-$ in equilibrium with MgCl$_2$ and AlCl$_3$. Knowledge of the stable species of the MACC electrolyte allows us to determine the most important equilibria occurring during electrochemical cycling. We observe that Al deposition is always preferred to Mg deposition, explaining why freshly synthesized MACC cannot operate and needs to undergo preparatory conditioning. Similarly, we suggest that aluminum displacement and depletion from the solution upon electrolyte resting (along with continuous MgCl$_2$ regeneration) represents one of the causes of electrolyte aging. Finally, we compute the NMR shifts from shielding tensors of selected molecules and ions providing fingerprints to guide future experimental investigations.
The capacity and stability of constituent electrodes determine the performance of Li-ion batteries. In this study, density functional theory is employed to explore the potential application of recently synthesized two dimensional phosphorene as electrode materials. Our results show that Li atoms can bind strongly with phosphorene monolayer and double layer with significant electron transfer. Besides, the structure of phosphorene is not much influenced by lithiation and the volume change is only 0.2%. A semiconducting to metallic transition is observed after lithiation. The diffusion barrier is calculated to 0.76 and 0.72 eV on monolayer and double layer phosphorene. The theoretical specific capacity of phosphorene monolayer is 432.79 mAh/g, which is larger than other commercial anodes materials. Our findings show that the high capacity, low open circuit voltage, small volume change and electrical conductivity of phosphorene make it a good candidate as electrode material.
In search of better thermoelectric materials, we have systematically investigated the thermoelectric properties of a 122 Zintl phase compound EuCd$_{2}$As$_{2}$ using textit{ab-initio} density functional theory and semi-classical Boltzmann transport theory within constant relaxation time approximation. Considering the ground state magnetic structure which is A-type antiferromagnetic (A-AFM) and non-magnetic (NM) structure, we evaluated various thermoelectric parameters such as Seebeck coefficient, electrical and thermal conductivity, power factor and figure of merit (ZT) as function temperature as well as chemical potential. Almost all thermoelectric parameters show anisotropy between $xx$ and $zz$ directions which is stronger in case of A-AFM than in NM. Both A-AFM and NM phase of the compound display better thermoelectric performance when hole doped. We observed high Seebeck coefficient and low electronic thermal conductivity in A-AFM phase along $zz$ direction. The remarkably high ZT of 1.79 at 500 K in A-AFM phase and ZT$sim$1 in NM phase suggest that EuCd$_{2}$As$_{2}$ is a viable thermoelectric material when p-doped.
Bismuth has recently attracted interest in connection with Na-ion battery anodes due to its high volumetric capacity. It reacts with Na to form Na$_3$Bi which is a prototypical Dirac semimetal with a nontrivial electronic structure. Density-functional-theory based first-principles calculations are playing a key role in understanding the fascinating electronic structure of Na$_3$Bi and other topological materials. In particular, the strongly-constrained-and-appropriately-normed (SCAN) meta-generalized-gradient-approximation (meta-GGA) has shown significant improvement over the widely used generalized-gradient-approximation (GGA) scheme in capturing energetic, structural, and electronic properties of many classes of materials. Here, we discuss the electronic structure of Na$_3$Bi within the SCAN framework and show that the resulting Fermi velocities and {it s}-band shift around the $Gamma$ point are in better agreement with experiments than the corresponding GGA predictions. SCAN yields a purely spin-orbit-coupling (SOC) driven Dirac semimetal state in Na$_3$Bi in contrast with the earlier GGA results. Our analysis reveals the presence of a topological phase transition from the Dirac semimetal to a trivial band insulator phase in Na$_{3}$Bi$_{x}$Sb$_{1-x}$ alloys as the strength of the SOC varies with Sb content, and gives insight into the role of the SOC in modulating conduction properties of Na$_3$Bi.
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