Simulating Materials in Devices --- Considering the Electronic and Ionic Freedom at the Solid-Electrolyte and Electrode Interfaces

Yue Qi*

School of Engineering, Brown University, Providence, RI, 02912, USA

ABSTRACT: Designing Materials means designing materials while they function in a device and interface with other materials. Energy storage devices, such as solid-state batteries and fuels are such example, in which the solid-state electronic heterojunctions at the solid-electrolyte and electrode interfaces play a critical role in determining ionic transport, and therefore the performance of devices. The formation of a “space-charge layer” at the solid electrolyte and electrode interface is often cited as a barrier for ion transport, but a comprehensive first-principles modeling approach is still missing. For solid-state-batteries (SSB), predicting the space-charge-layer formation at the solid-electrolyte-and-electrode interface is even more challenging, as ion insertion and/or reaction with the electrode alters the material and thus band alignments at the interfaces. In this talk, a theoretical framework was established to predict the interface potential profiles from thermodynamic driving forces. We first assumed the electrochemical potential for Li⁺ ions reached a constant at the open circuit equilibrium condition, then derived the relationship among the electrostatic potential, the lithium chemical potential, Fermi level, ionization potential, and the work function. This relationship yielded quantitative profiles of the electrostatic potential and electronic energy level alignments across the entire battery. This model complemented direct microscopic and macroscopic simulations by rigorously and simultaneously determining the potential drop, electrostatic dipole, and space-charge layer at the interface. The application of this model to the Li/LiPON/Li_xCoO₂ system led to the important discovery that the space-charge layer varies with the state of charge (SOC, i.e. Li concentration in LixCoO₂). This modeling framework is general for other solid-state energy storage devices with ionic and mixed conductor interfaces. It highlights the importance of predicting the functionalities of materials while they are in a device.

Keywords：first principles, thermodynamics, space charge layer, solid state batteries, interfaces.