UC San Diego

Understanding the structure and diffusion properties in photovoltaic and battery materials is imperative to understanding the device stability and performance. In this work we investigate structure-diffusion relationships in two complementary energy technologies – lead halide perovskites (LHPs) solar cells and lithium–sulfur batteries (LSBs) – using ab initio calculations and machine learning inter atomic potentials (MLIAPs).

LHP solar cells have seen a meteoric rise in their power conversion efficiency (>25%) in the last decade. However, halide ion migration induced instability and hysteresis is a major impediment to its commercialization. In the first project, we demonstrate the use of constrained nudged elastic band (NEB) calculations to quantitatively establish the correlated effects of A site cation motion, H bonding and octahedral rotation on halide ion migration in APbBr3 (A = Cs or methylammonium/MA) LHPs. LSBs are among the most promising energy storage solutions due to their high theoretical capacity (1672 mAh g-1) and the low cost of S but suffers from polysulfide shuttling induced capacity fading. Replacing traditional liquid electrolytes with solid-state electrolytes (SEs) is a potential solution to this problem. In the second project, we investigated the (electro)chemical stability of cathode/SE interfaces in LSB using a combination of DFT and ML-IAPs based on the moment tensor potential (MTP) formalism. Thermodynamic analysis predicts that among the major SE chemistries (oxides, sulfides, nitrides, and halides), sulfides are generally predicted to be the most stable against the S8 cathode. Molecular dynamics (MD) simulations on realistic large-scale (thousands of atoms) α-S8|β-Li3PS4 interfaces using the developed MTP reveal the formation of LixSy and Sx species at the interface. The reaction products were in general found to increase the activation energy (Ea) of Li-ion diffusion at the interface, but the type of β-Li3PS4 surface interfaced was found to predominant determine the interfacial Li-migration activation barriers.

The other critical issues of LSBs are the low electronic conductivity of S which limits practical capacity and the high interfacial impedance induced by large volume change (>80%) during cycling. In the third part we report an electrically conducting low melting point (65°C) S9.3I cathode which is demonstrated to retain 87% capacity after 400 cycles with periodic reheating that facilitates interfacial contact healing. We use DFT calculations to elucidate the hitherto-unknown structure of this cathode and understand the origin of its high electrical conductivity.