A first principles investigation of alternative energy materials for power generation and storage

Abstract

Before renewable energy can completely replace the traditional power grid cheaper and more efficient energy generation and storage materials must be developed. Firstprinciples simulations based on density functional theory (DFT) are a valuable tool to study the properties of known materials and propose new materials with energy application. Al-ion batteries offer a promising storage alternative to Li-ion batteries as a result of Al’s superior volumetric capacity, high specific capacity, stability, and abundance. However, Al’s trivalency has made development of suitable cathodes challenging. Two recent promising cathodes are graphite and Chevrel phase Mo6S8. The behavior of each battery system is studied using DFT simulations. The graphite cathode allows thermodynamically unstable intercalation of the AlCl4 anion through interaction with the ionic liquid electrolyte. The system can rapidly charge and discharge due to low diffusion barrier of AlCl4 in graphite and exhibits much larger voltages than alternative Al-ion batteries since monovalent AlCl4 is the intercalated species. The Mo6S8 cathode has previously been used as a cathode in Mg and Li battery systems. When used with an Al anode there has been discrepancy in the final discharge product of the cathode after Al intercalation. Comparing the thermodynamics of Al intercalation with the reported lattice constants of the cathode after cell discharge indicates that Al4/3Mo6S8 is the final discharge product. This is confirmed by the electronic structure of the cathode since a metal to semiconductor transition is observed at an Al concentration of 4/3. Al4/3Mo6S8 has a 1.18 eV indirect gap and 1.35 eV direct gap indicating its potential use as an absorber material in a photovoltaic cell.

Over the past 5 years perovskite photovoltaic cells have stimulated significant research interest. The breakthrough was based on organic-inorganic CH3NH3PbI3 which offers high efficiency with low material and synthesis cost. A DFT study on the point defects in this material show defect tolerance, which is rare when low cost synthesis methods are used. However, CH3NH3PbI3 suffers from intrinsic instability with carcinogenic PbI2 as a decay product, making a replacement necessary. Alternative materials in the perovskite structure are explored to find potential replacements using a combination of machine learning and Edisonian approaches. Trivalent cations such as Sb or Bi can be accommodated by splitting the anions in the perovskite to combinations of halides and chalcogenides and almost the entire periodic table can be searched by splitting cations in the perovskite structure with chemical formula A2BB’X6.