High-Pressure Low-Temperature Characterization of ZnPS₃ for Energy Applications

We present an experimental and computational study of high-pressure and cryogenic-temperatures properties of ZnPS₃. Belonging to the metal thio(seleno)phosphate family (MPX₃, X = S, Se) of layered materials, ZnPS₃ has been identified as an exceptional inorganic electrolyte for solid-state batteries, which are more environmentally friendly than conventional lithium batteries. Its potential is attributed to its high ionic conductivity with a low diffusion energy barrier, high electrochemical stability, and mechanical strength, which can be enhanced in the presence of water vapor [1,2]. Furthermore, ZnPS₃ has significant photocatalytic activity for hydrogen generation, given its dimensional tunability and plentiful active <i>P</i> and <i>S</i> sites that facilitate hydrogen adsorption and desorption [3,4].<br/>Our research on ZnPS₃ delves into its phase and thermal stability. We employ high-pressure (HP) low-temperature (LT) Raman and photoluminescence (PL) spectroscopy, supported by first-principle calculations using density functional theory (DFT). The HP Raman data reveals two minor phase transitions at ~6.75 GPa and ~12.5 GPa. Our DFT calculations extend the study to hydrostatic pressures beyond the experimentally feasible range and predict a semiconductor-to-semimetal transition at ~100 GPa, confirming reliable semiconductor behavior under extreme conditions.<br/>Temperature-dependent properties are estimated using the quasi-harmonic approximation, combining pressure and temperature effects [2]. The anticipated changes in the band gap align with those derived from low-temperature absorption measurements. X-ray diffraction at cryogenic temperatures allowed us to calculate the thermal expansion coefficient for ZnPS₃ and compare it with known semiconductors based on ionic conductivity and band gap. Our calculations show that ZnPS₃ has a relatively high band gap, which is modestly tunable by strain, crucial for preventing electronic leakage in solid-state batteries. ZnPS₃ strikes a good balance between high ionic conductivity and minimal electronic leakage. Additionally, its low thermal expansion coefficient revealed in our study enhances its suitability for energy storage applications. The combination of divalent ionic conduction and low thermal expansion in ZnPS₃ offers increased energy density, improved stability and safety, and consistent performance across various temperatures. These properties make ZnPS₃ a promising material for next-generation solid-state batteries.<br/>This work has been supported by the MIT-Poland Lockheed Martin Seed Fund, the ARO MURI Grant No. W911NF-19-1-0279 and MIT-Tecnologico de Monterrey Collaborative Research Program in Nanotechnology. Abhishek Mukherjee appreciates the support provided by the Siebel Scholarship and MIT Mathworks Fellowship. Michael A. Susner acknowledges the support of the Air Force Office of Scientific Research (AFOSR) Grant No. LRIR 23RXCOR003 and AOARD MOST Grant No. F4GGA21207H002 and general support from the Air Force Materials and Manufacturing (RX) and Aerospace Systems (RQ) Directorates. Vivian Santamaría-García acknowledges the support of Tecnologico de Monterrey and the MIT SuperCloud and Lincoln Laboratory Supercomputing Center for resources that have contributed to the research results reported within this work.The Polish National Science Center SHENG-2 Grant No. 2021/40/Q/ST5/00336 also partially supported this project.<br/>*Abhishek Mukherjee and Vivian Santamaria-Garcia contributed equally to this work.<br/>References:<br/>[1]<i> Adv. Funct. Mater</i>. <b>34</b>, 2310476 (2024).<br/>[2] <i>Chem. Mater.</i> <b> 31</b>, 10, 3652–3661 (2019).<br/>[3] <i>J. Mater. Chem. A</i>, <b>11</b>, 16933 (2023).<br/>[4] <i>Materials Letters</i> <b>324, </b>132687<b> </b>(2022).