Batteries, especially with the advent of the state-of-the-art Li-ion architecture, have become ubiqutous as energy storage devices for a range of applications, from portable electronics and electric vehicles to grid-scale storage. However, given the supply-chain and geo-political constraints associated with some of the elements critical in making the modern Li-ion battery, it is imperative to explore frameworks that can achieve similar energy densities as Li-ion with more earth-abundant elements. Promising beyond Li-ion architectures include the development of Na-ion and multivalent (based on Mg-ion for example) batteries.

All battery frameworks that go beyond Li-ion require robust materials, either as electrodes that can “intercalate” the active ion reversibly, and/or electrolytes that are stable against both electrodes. Specifically, solid electrolytes are of significant interest in the development of beyond Li-ion batteries owing to their safety. Another important requirement in developing new battery frameworks is to develop better interfaces (or coating layers) that exist between electrodes and electrolytes. Thus, this field of research will include discovery of new materials, such as high-voltage negative electrodes, robust ionic conductors and stable interfaces, while also understanding the behavior of existing candidates, using density functional theory (and beyond) calculations.

References

1. Canepa et al., “Odyssey of multivalent cathode materials: open questions and future challenges”, Chem. Rev. 117, 4287-4341 (2017). DoI.
   
2. Sai Gautam et al., “Influence of inversion on Mg mobility and electrochemistry in spinels”, Chem. Mater. 29, 7918-7930 (2017). DoI.
   
3. Butler et al., “Designing interfaces in energy materials applications with first-principles calculations”, npj Comput. Mater. 5, 19 (2019). DoI.

Photovoltaics (or PVs) are an important component towards creating a sustainable, fossil-free energy future. Currently, the PV market is dominated by single-junction solar cells made of crystalline Si, benefiting from the decades of research and calibration that went into developing Si-based transistors and computing chips. However, the performance of Si-based solar cells is inhibited by the intrinsic electronic properties of Si (e.g., indirect band gap), which typically requires the production of high-purity Si and consequently drives up the solar cell costs. Hence, developing PVs that do not use Si as the base semiconductor can result in both better efficiencies and lower cost.

Among the various beyond-Si materials that have been studied for PV applications, Cu₂ZnSnS₄ (CZTS) displays significant promise, since it’s constituents are earth-abundant, it exhibits a near-optimal band gap for PV, and it can be produced using cheap wet-chemistry methods. However, CZTS suffers from intrinsic defect formation (e.g., Cu_Zn+Zn_Cu, Sn_Zn+2Cu_Zn antisite clusters) that typically reduces its performance, via inducing electrostatic potential fluctuations (reducing the voltage), and/or trap states that lie deep within the band gap (reducing the free carrier concentration or current). Doping and/or alloying CZTS has been shown to suppress some of the defect formation in prior studies. Hence, this research direction will focus on developing doping and/or alloying additions, using density functional theory (and beyond) calculations, that can aid in improving the performance of CZTS-based solar cells by suppressing detrimental defects. This direction will also explore “new” chemistries that have the potential to be better than Si for PV applications.

References

1. Sai Gautam et al., “Novel solar cell materials: insights from first-principles”, J. Phys. Chem. C 122, 27107-27126 (2018). DoI.
   
2. Wexler et al., “Exchange-correlation functional challenges in modeling quaternary chalcogenides”, Phys. Rev. B 102, 054101 (2020). DoI.
   
3. Wallace et al., “Atomistic insights into the order-disorder transition in Cu₂ZnSnS₄ solar cells from Monte Carlo simulations” J. Mater. Chem. A 7, 312-321 (2019). DoI.

Amorphous solids are important for a variety of applications, including optical (lenses), structural (window glasses, thermal insulation), magnetic (bulk metallic glasses), and art (stained glasses). Recently, amorphous solids have gained prominence in energy applications, such as amorphous Si photovoltaic cells, amorphous Li- Si alloy electrodes in Li-ion batteries, and glassy lithium phosphorus oxy-nitride (LiPON) electrolytes in thin-film batteries for microelectronics. However, more studies are required to understand both the thermodynamic (glass transition temperature – T_g, Kauzmann temperature – T_K, structure) and kinetic (dynamic structure factors, self-diffusion) properties of glasses. From a materials-engineering perspective, predicting the ability of a given composition to form an amorphous phase can lead to the identification of new glassy compounds with potentially important applications.

While this research direction will look at a few select systems in the short future, the eventual aim will be to construct a general theoretical framework, and address the question, “Is it feasible for a given composition to form a ‘useful’ glass under practical cooling rates?”.

References

1. Lacivita et al., “Resolving the amorphous structure of lithium phosphorus oxynitride (LiPON)”, J. Am. Chem. Soc. 140, 11029-11038 (2018). DoI.
   
2. Sivonxay et al., “The lithiation process and Li diffusion in amorphous SiO₂ and Si from first-principles”, Electrochim. Acta 331, 135344 (2020). DoI.
   
3. Xia and Carter, “Orbital-free density functional theory study of amorphous Li-Si alloys and introduction of a simple density decomposition formalism”, Modelling Simul. Mater. Sci. Eng. 24, 035014 (2016). DoI.