arXiv subjects

Dereje Bekele Tekliye

Publications and source records attributed to Dereje Bekele Tekliye.

4 recordsLinked to original sources

arXiv2026

Phase stability and ionic transport in post-spinel CaV$_2$O$_4$ cathode

Calcium-ion batteries (CBs) represent an alternative to lithium-ion technology but their advancement is limited by the lack of high-performance intercalation cathodes. Identified via computational screening, post-spinel CaV$_2$O$_4$ has emerged as a promising candidate, though its practical application is hindered by limited electrochemical capacity. Hence, we investigate the thermodynamic and ionic transport characteristics of Ca$_x$V$_2$O$_4$ ($0 \leq x \leq 1$) in this work, by integrating the cluster expansion formalism with Monte Carlo simulations and density functional theory based calculations. We construct the temperature-composition phase diagram of Ca$_x$V$_2$O$_4$ revealing several stable phases ($α$ through $ζ$) that can appear during electrochemical operations at different voltages. Importantly, we observe the formation of the $\varepsilon$ phase at $x \sim 0.83$ across a 370-590~K temperature window via invariant reactions, which agrees with observations in the experimental voltage profiles. Further, migration barrier calculations confirm that Ca mobility is severely impeded within the $α$ ($x \sim 0$) and $γ$ ($x \sim 0.5$) phases. With the strong Ca-vacancy ordering contributing to the high barrier in $γ$ and the persistent two-phase region stretching across the $δ$ ($x \sim 0.67$) and the $γ$ phases, we expect the accessible electrochemical capacity in the CaV$_2$O$_4$ system to be kinetically limited to at most half the theoretical capacity at 298~K, in agreement with experiments. Strategies including cation doping and particle size reduction can be considered to flatten the potential energy landscape of $γ$ and improve Ca mobility. Our computational findings highlight the interplay between stability and transport and provide design strategies that can enable the practical use of CaV$_2$O$_4$ as a CB cathode.

cond-mat.mtrl-sci↗

arXiv2026

Geometry-based Discovery of Calcium Battery Cathodes Accelerated by Foundational Machine-Learned Models

Calcium batteries (CBs) are an attractive post-Li-ion technology, offering the appeal of Ca's natural abundance and high volumetric energy density. However, practical realization of CBs remains limited by the scarcity of positive electrode (cathode) materials that support reversible Ca$^{2+}$ (de)intercalation under electrochemical conditions. To address this challenge, we screen the materials project (MP) database for novel host structures that can intercalate Ca using geometry- and chemistry-based design principles. Specifically, we employ the Voronoi polyhedral volume as a descriptor of site compatibility for hosting Ca in potential frameworks. Further, we down-select candidate structures progressively through criteria including charge neutrality, absence of non-Ca mobile cations, thermodynamic (meta)stability, average voltage, and Ca migration barriers ($E_m$) using foundational machine-learning (ML) models. Subsequently, we validate the ensemble-ML-predicted $E_m$ in a subset of the final candidates using density functional theory based nudged elastic band calculations. Overall, from an initial pool of 52,945 MP structures, our workflow identifies 37 promising Ca cathode candidates, several of which exhibit favorable combinations of thermodynamic (meta)stability, voltage, and Ca-mobility, marking them as strong candidates for synthesis and electrochemical characterization. Particularly, we identify two Ca cathode candidates with markedly low Ca$^{2+}~E_m$ (CaSc$_2$V$_2$O$_8$ and CaVSO$_4$F$_3$), and four cathode candidates with thermodynamically stable charged states (Ca$_3$(CoO$_2$)$_4$, Ca$_3$Mn$_4$(TeO$_6$)$_2$, CaVF$_5$, and CaVSO$_4$F$_3$). Beyond identifying Ca-cathodes, our work establishes geometry-based descriptors and ML-based workflows as transferable methods for high-throughput screening, enabling the rapid discovery of novel materials for battery and other applications.

cond-mat.mtrl-sci↗

arXiv2024

Fluoride Frameworks as Potential Calcium Battery Cathodes

Calcium batteries (CBs) are potential next-generation energy storage devices, offering a promising alternative to lithium-ion batteries due to their theoretically high energy density, better safety, and lower costs associated with the natural abundance of calcium. However, the limited availability of positive electrode (cathode) materials has constrained the development of CBs so far. Given the similar ionic radii of Na$^+$ and Ca$^{2+}$, structures that are effective at reversibly intercalating Na$^+$ may be able to reversibly intercalate Ca$^{2+}$ as well. In this context, transition metal fluorides (TMFs) exhibiting weberite and perovskite structures that are known for intercalating Na$^+$ form an interesting set of possible CB cathode frameworks. Thus, we use first principles calculations to explore weberite and perovskite TMFs as CB cathodes, of compositions Ca\textsubscript{x}M\textsubscript{2}F\textsubscript{7} and Ca\textsubscrpt{x}MF\textsubscript{3}, respectively, where M = Ti, V, Cr, Mn, Fe, Co, or Ni. We systematically evaluate key cathode properties, including ground state structure, average Ca-intercalation voltage, thermodynamic stability (at 0 K), theoretical capacity, and Ca$^{2+}$ migration barriers. Importantly, we identify Ca\textsubscript{x}Cr\textsubscript{2}F\textsubscript{7} and Ca\textsubscript{x}Mn\textsubscript{2}F\textsubscript{7} weberite frameworks as promising Ca-cathodes. Our study not only unveils potential CB cathodes but also paves the way for further advancement in TMF-based intercalation cathodes, diversifying the chemical space for next-generation energy storage systems.

cond-mat.mtrl-sci↗

arXiv2024

Accuracy of metaGGA functionals in describing transition metal fluorides

Accurate predictions of material properties within the chemical space of transition metal fluorides (TMFs), using density functional theory (DFT) is important for advancing several technological applications. The state-of-the-art semi-local exchange-correlation functionals within DFT include the strongly constrained and appropriately normed (SCAN) and the restored regularized SCAN (r$^2$SCAN), both of which are meta generalized gradient approximation (metaGGA) functionals. Both SCAN and r$^2$SCAN are susceptible to self-interaction errors (SIEs) while modelling correlated $d$ electrons of transition metals. Hence, in this work, we evaluate the accuracy of both functionals in estimating properties of TMFs, including redox enthalpies, lattice geometries, on-site magnetic moments, and band gaps. We observe both SCAN and r$^2$SCAN to exhibit poor accuracy in estimating fluorination enthalpies among TMFs, attributable to SIEs among the $d$ electrons. Thus, we derive optimal Hubbard $U$ corrections for both functionals based on experimental fluorination enthalpies of binary TMFs. Note that the linear response theory yielded unphysical $U$ values for V, Fe, and Ni fluorides. While adding the optimal $U$ to the metaGGA functionals does not significantly affect the lattice volumes and magnetic moments, it does significantly increase the calculated band gaps. Also, we calculate the average Na intercalation voltage in Mn, Fe, Co, and Ni fluorides as a transferability check of our optimal $U$ values. Overall, we recommend using the Hubbard $U$ correction to improve predictions of redox enthalpies in other TMFs, while for band gap predictions, we suggest using the non-corrected functionals. Finally, our study should advance the accuracy of DFT-based screening studies to unearth novel TMFs, which can be used in various applications, including energy storage, catalysis, and magnetic devices.

cond-mat.mtrl-sci↗