Session Y19: Computational Approaches for Energy Materials

Beller Lectureship: Materials for Li {\&} Na Batteries :A Computational Materials Science Point of View

Energy storage has been a theme for scientists for two hundred years. The Lead acid battery research on batteries occupied some of the best minds of 19th century. Plante in 1859 invented lead acid battery which starts your car and ignites internal combustion which takes over the propulsion. Although the lead battery is over 150 years old but the origin of its open circuit voltage (OCV) of 2.1 V is still known. In present talk, I will show how one can explain the origin of OCV of 2.1 V based on foundations of relativistic quantum mechanics. Surprisingly, seems to be the first time its chemistry has been theoretically modeled from the first principles. The main message of this work is that most of the electro-motoric force of the common lead battery comes from relativistic effects. In second part, I will provide an overview of the most recent theoretical studies undertaken by us in the field of materials for Li {\&} Na ion batteries. For selected examples, I will show how ab initio calculations can be of use in the effort to reach a better understanding of battery materials and to occasionally also guide the search for new promising materials.   

Rational design of nontoxic electrolytes for metal-ion batteries

Most of the electrolytes used in current Li-ion batteries contain halogens, which are both toxic and corrosive. In an effort to search for halogen-free electrolytes, we studied the electronic structure of these complexes using density functional theory and found that the negative ions of all the current electrolytes are superhalogens, i.e., the vertical electron detachment energies of these moieties are larger than that of any halogen atom. Realizing that several superhalogens that do not contain even a single halogen atom exist, we studied their potential as effective electrolytes by calculating not only the energy needed to remove a Li$^{\mathrm{+}}$ ion but also their affinity towards H$_{\mathrm{2}}$O. Several halogen-free electrolytes are identified among which Li(CB$_{\mathrm{11}}$H$_{\mathrm{12}})$ was shown to have the greatest potential. Replacing H in Li(CB$_{\mathrm{11}}$H$_{\mathrm{12}})$ with CN or SCN moieties further improves the electrolyte performance. Validation of our prediction by recent experiments as well as a new family of super-ion inspired solid electrolytes based on anti-perovskite structures will also be discussed. More information can be found from S. Giri, S. Behera, and P. Jena, Angew. Chem. Int. Ed. \textbf{53}, 13916 (2014) and H. Zhao, J. Zhou, and P. Jena, Angew. Chem. Int. Ed. (\textit{VIP}) \textbf{55}, 3704 (2016).   

Discovery of new solar fuels photoanode materials with a combination of high-throughput theory and experiment

The discovery and design of new complex functional materials -- and an understanding of their emergent phenomena and functional behavior in terms of their chemical composition and atomic-scale structure -- is a grand challenge. In particular, the dearth of known low-band-gap photoelectrocatalytic materials poses roadblocks for the efficient generation of chemical fuels from sunlight. In this talk, I will describe a new pipeline that integrates high-throughput \textit{ab initio} density functional theory calculations with high-throughput experiments. Our pipeline has led to the rapid identification of 12 ternary vanadate oxide photoelectrocatalysts for water oxidation, doubling the number of known photoanodes in the band gap range 1.2-2.8 eV, and establishing these vanadates as the most prolific class of photoanode materials for generation of chemical fuels from sunlight. Additionally, our calculations reveal new correlations between the VO$_{\mathrm{4\thinspace }}$structure motif, d electron configuration, and electronic band edge character of these oxides. Accordingly, I will discuss how this work could initiate a `genome' for photoanode materials and future applications of our high-throughput theory-experiment pipeline for materials discovery.   

Intercalating layered materials for energy storage

Layered materials are widely used as energy-storage media in applications such as hydrogen storage and batteries. Computational approaches can provide valuable insights into the underlying storage mechanisms and shed light on strategies to improve materials performance. We have employed advanced hybrid functional calculations to study two types of intercalated layered materials: (1) hydrogen-intercalated MoS$_2$ and (2) sodium-intercalated MnO$_2$. Our goal is to elucidate intrinsic materials properties that affect energy storage. \\ \\ We have studied the interactions of hydrogen with MoS$_2$ by exploring the equilibrium geometry, formation energy, and electronic behavior of interstitial H and H$_2$ molecules inside layered MoS$_2$ structures [1]. Interstitial H is identified to be a deep donor while H$_2$ molecules are electrically inactive and energetically more stable in MoS$_2$. To further shed light on the hydrogen-storage capacity of MoS$_2$, we have also explored the insertion energies of H$_2$ molecules as a function of hydrogen concentration and found that up to 13 H$_2$ molecules can be accommodated within the same interlayer spacing of an areal $3\times3$ supercell. \\ \\ In the second part of the talk, I will discuss electronic and ionic conductions in layered NaMnO$_2$, a cathode material for sodium ion batteries. Free carriers are trapped to form small hole or electron polarons; hence, electronic conduction is through polaron hopping. Ionic conduction is in the form of sodium vacancy migration. Both electronic and ionic conduction can be significantly affected by the presence of point defects. We will discuss strategies, such as optimizing synthesis conditions and impurity doping, to improve electrical conduction and storage performance of NaMnO$_2$. \newline\newline The work was performed in collaboration with H. Peelaers and C. G. Van de Walle, and supported by DOE. \newline\newline [1] Z. Zhu, H. Peelaers, and C. G. Van de Walle, Phys. Rev. B 94, 085426 (2016).   

Defect physics as key to understanding complex battery electrode materials

In complex functional materials such as those for metal-ion battery electrodes, point defects can be vital or fatal to the performance. A detailed understanding of the formation and migration of these defects is thus required for explaining, predicting, and optimizing the materials' properties, and for rational materials design. With advances in electronic-structure methods, first-principles calculations for defects have become a powerful tool in providing such an understanding. In this talk, I will focus my discussion on defect physics vis-\`{a}-vis functional properties in mixed ionic-electronic conducting, electrode materials. Specific examples will be taken from recent work on complex transition-metal oxides. Through these examples, I will illustrate how state-of-the-art point defect calculations can serve as a study of materials’ response to interventions, done on purpose and in a well-controlled manner, at the electronic and atomic level, and how such a study can provide a fundamental understanding of the materials and help uncover new science.