DFT investigation of Ca mobility in reduced-perovskite and oxidized-marokite oxides

Progress in the development of rechargeable Ca-ion batteries demands the discovery of potential cathode materials. Transition metal oxides are interesting candidates due to their theoretical high energy densities, but with the drawback of a low Ca mobility. Previous computational/experimental investigations associate the electrochemical inactivity of various oxides (CaMO3-perovskite, CaMn2O4-post-spinel and CaV2O5) to high energy barriers for Ca migration. The introduction of oxygen and/or Ca vacancies in ternary transition metal oxides is a likely way to reshape the local topology and hence improve the Ca diffusivity. In this work, the energy barriers for Ca migration are calculated and discussed for (i) oxygen-deficient perovskites within the related Ca2Fe2O5-brownmillerite and Ca2Mn2O5 structures, and (ii) tunnel CaMn4O8, a derivative of the CaMn2O4-marokite with Ca vacancies

Electronic Structure of the Complex Hydride NaAlH4

Density functional calculations of the electronic structure of the complex hydride NaAlH4 and the reference systems NaH and AlH3 are reported. We find a substantially ionic electronic structure for NaAlH4, which emphasizes the importance of solid state effects in this material. The relaxed hydrogen positions in NaAlH4 are in good agreement with recent experiment. The electronic structure of AlH3 is also ionic. Implications for the binding of complex hydrides are discussed.Comment: 4 pages, 5 figure

Comparative computational investigation of N and F substituted polyoxoanionic compounds: The case of Li 2FeSiO 4 electrode material

First principles calculations are used to anticipate the electrochemistry of polyoxoanionic materials consisting of XO 4 - yA y (A = F, N) groups. As an illustrative case, this work focuses on the effect of either N or F for O substitution upon the electrochemical properties of Li 2FeSiO 4. Within the Pmn2 1-Li 2FeSiO 4 structure, virtual models of Li 2Fe 2 2.5+SiO 3.5N 0.5 and Li 1.5Fe 2+SiO 3.5F 0.5 have been analyzed. We predict that the lithium deinsertion voltage associated to the Fe 3+/Fe 4+ redox couple is decreased by both substituents. The high theoretical specific capacity of Li 2FeSiO 4 (330 mAh/g) could be retained in N-substituted silicates thanks to the oxidation of N 3- anions, whilst Li 1.5Fe 2+SiO 3.5F 0.5 has a lower specific capacity inherent to the F substitution. Substitution of N/F for O will respectively improve/worsen the electrode characteristics of Li 2FeSiO 4. © 2011 Elsevier B.V. All Rights Reserved

On-demand design of polyoxianionic cathode materials based on electronegativity correlations: An exploration of the Li2MSiO4 system (M = Fe, Mn, Co, Ni)

A first principles investigation is performed to quantify how the inductive effect of different polyoxianions (XO4)n- (X = Ge, Si, Sb, As, P) affects the lithium deintercalation voltage of olivine-LiCo+2XO4, LiyV+4OXO4 and LiyM+2XO4 (M = Mn, Fe, Co, Ni within the structure of Li2FeSiO4) compounds. In all cases, the calculated lithium deintercalation voltage correlates to the Mulliken X electronegativity, displaying a linear dependence for each structural type/redox couple. Experimental lithium deinsertion voltages already available in the literature support these results. Computational results on Li2MSiO4 are used to evaluate the electrochemical performance of these materials. Li2FeSiO4 will develop a reversible specific capacity limited to the extraction of one lithium ion. Li2MnSiO4 is predicted to have a poor electronic conductivity. The calculated lithium extraction voltages of Co and Ni silicates are too high for current electrolytes. The optimized structure of the fully delithiated MSiO4 suggests that this host is unstable, pointing out to a possible structural transformation during the charge/discharge of the lithium cells. © 2006 Elsevier B.V. All rights reserved

Electrochemical data transferability within LiyVOXO4 (X = Si, Ge0.5Si0.5, Ge, Si0.5As0.5, Si0.5P0.5, As, P) polyoxyanionic compounds

Given the interest of silicates as potential electrode materials for lithium batteries, it is critical to fully understand the role that the inductive effect of the polyoxyanionic group, (XO4)n-, plays on the electrochemical performance of polyoxyanionic compounds. In this work we have combined experiments and first-principles methods to investigate to which extent the inductive effect of the X-O bond within the XO y n- polyanion (X = Si, Ge0.5Si0.5, Ge, Si0.5As0.5, Si0.5P0.5, As, P) modifies the redox energy of the V5+/V4+ couple in the Li0.5VOXO4 family of compounds. Calculations using the GGA+U method evidence a nice correlation between the X electronegativity (i.e., the magnitude of the XO4 groups' inductive effect) and details of the crystalline and electronic structures of Liy+1V4+OXO 4/ Liy,V5+OXO4 phases such as bond distances or band gaps. Besides the inductive effect of the polyanionic group, we found that the chemistry of these 2D compounds is also correlated to the Li+ site occupancy in the interlayer space. The calculated lithium insertion voltages display an almost linear dependence on the Mulliken X electronegativity; this offers promising prospects in the design of novel polyoxyanionic electrode materials. The new electrode materials Li 2VOSi4 and Li2VOSi0.5Ge 0.5O4 have been electrochemically tested, providing good agreement between experimental and calculated lithium insertion voltages. Activation energies of the prepared compounds follow the band gap trend provided by the calculated data as well. Thus, experimental evidence support the computational results and the conclusions presented here. © 2007 American Chemical Society

Analysis of Minerals as Electrode Materials for Ca-based Rechargeable Batteries

Rechargeable lithium-ion batteries dominate the consumer electronics and electric vehicle markets. However, concerns on Li availability have prompted the development of alternative high energy density electrochemical energy storage systems. Rechargeable batteries based on a Ca metal anode can exhibit advantages in terms of energy density, safety and cost. The development of rechargeable Ca metal batteries requires the identification of suitable high specific energy cathode materials. This work focuses on Ca-bearing minerals because they represent stable and abundant compounds. Suitable minerals should contain a transition metal able of being reversibly reduced and oxidized, which points to several major classes of silicates and carbonates: olivine (CaFeSiO; kirschsteinite), pyroxene (CaFe/MnSiO; hedenbergite and johannsenite, respectively), garnet (CaFe/CrSiO; andradite and uvarovite, respectively), amphibole (CaFeSiO(OH); ferroactinolite) and double carbonates (CaMn(CO); kutnahorite and CaFe(CO); ankerite). This work discusses their electrode characteristics based on crystal chemistry analysis and density functional theory (DFT) calculations. The results indicate that upon Ca deintercalation, compounds such as pyroxene, garnet and double carbonate minerals could display high theoretical energy densities (ranging from 780 to 1500 Wh/kg) with moderate structural modifications. As a downside, DFT calculations indicate a hampered Ca mobility in their crystal structures. The overall analysis then disregards olivine, garnet, pyroxene, amphibole and double carbonates as structural types for future Ca-cathode materials design.Funding from the European Union’s Horizon 2020 research and innovation programme H2020 FETOPEN-1-2016-2017 (CARBAT, grant agreement no. 766617) is acknowledged. The authors are grateful for access to the computational facilities from Universidad de Oviedo (MALTA-Consolider cluster

Assessing Si-based anodes for Ca-ion batteries: Electrochemical decalciation of CaSi2

Density functional theory (DFT) calculations are used to investigate the basic electrochemical characteristics of Si-based anodes in calcium ion batteries (CIBs). The calculated average voltage of Ca alloying with fcc-Si to form the intermetallic CaxSi phases (0.5 < x ≤ 2) is of 0.4 V, with a volume variation of 306%. Decalciation of the lower Ca content phase, CaSi2, is predicted at an average voltage between 0.57 V (formation of Si-fcc, 65% volume variation) and 1.2 V (formation of metastable deinserted-Si phase, 29% volume variation). Experiments carried out in conventional alkyl carbonate electrolytes show evidence that electrochemical “decalciation” of CaSi2 is possible at moderate temperatures. The decalciation of CaSi2 is confirmed by different characterization techniques. Keywords: Ca-ion batteries, Si anode, CaSi alloys, CaSi