Modeling of the Potential Distribution and the Electrical Double Layer in Solid-State Batteries

The electrical double layer (EDL) is a key feature of all electrochemical interfaces, controlling the kinetics and thermodynamics of both electron transfer and ion transfer reactions, and therefore the performance of all solid-state batteries (ASSB). Unlike in a liquid electrolyte with an EDL made of solvated ions, the EDL in a solid electrolyte is made up of charged point defects. Thus, the well-known Poisson-Boltzmann equation at electrode/liquid-electrolyte interfaces no longer holds in SSBs. This problem is even more challenging, as ion insertion and/or reaction with the electrode alters the material and thus band alignments at the electrode/solid-electrolyte interfaces.<br/>In this talk, a density functional theory (DFT)-informed theoretical framework was established to predict the interface potential profiles. 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 solid-state batteries.<br/>The electrostatic potential jump at the electrode/electrolyte interface creates intrinsic barriers for Li⁺ transport across the interface. The predicted potential jump was directly compared to the Kelvin probe force microscopy (KPFM) measurements and correlated with the interfacial impedance. The interfacial potential jump was then predicted for 48 ASSBs made of different electrolyte and electrode materials. A number of cathode/electrolyte pairs with low “intrinsic” potential barriers were identified, suggesting these promising interfaces will most benefit from engineering efforts to reduce “extrinsic” interfacial impedance, such as increasing the interface contact area.<br/>To obtain the electrostatic potential variation as a function of the distance to the interface, a more general model for the EDL at a solid-state electrochemical interface based on the Poisson-Fermi-Dirac equation was developed. The EDL structure is presented in various materials that are thermodynamically stable in contact with a lithium metal anode. The model further allows designing the optimum interlayer thicknesses to stabilize the interface without introducing additional electrostatic barriers for lithium ion transport at relevant solid-state battery interfaces.