Space charge layers and interface potentials in Li/γ−Li3PO4/LixCoO2 solid-state batteries: Insights…

Understanding the evolution of the space charge layer (SCL) at the electrode-solid electrolyte (SE) interface is crucial for elucidating the failure mechanism and addressing the rate performance bottleneck of all-solid-state batteries (ASSBs). However, discrepancies between current theories and experimental reports necessitate the urgent development of a comprehensive theoretical prediction. Herein, we present a comprehensive study of the defect formation energies, electronic band alignments, and potential profiles in $\mathrm{Li}/\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Li}}_{3}{\mathrm{PO}}_{4}/{\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ system to probe the SCL formation and its impact on Li-ion transport using a first-principles-informed thermodynamic model. We reveal the formation of Li defects in $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Li}}_{3}{\mathrm{PO}}_{4}$ and at the SE/electrode interfaces, and find that the dominant Li carrier in $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Li}}_{3}{\mathrm{PO}}_{4}$ changes with Li chemical potential within the battery. Through the analysis of defect formation energies and band alignments, we predict that the SCL, arising from the Li-ion and electron transfer, is responsible for the interface potential drops, influencing the Li-ion transport. In addition, we find that the largest potential drop occurs at the $\mathrm{Li}/\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Li}}_{3}{\mathrm{PO}}_{4}$ interface, which makes it a rate-limiting interface, hindering the discharge process. Furthermore, it is found that the interfacial potential drops depend strongly on both the battery's state of charge (SOC) and the crystal orientation. Interestingly, different $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Li}}_{3}{\mathrm{PO}}_{4}$ crystal orientations determine initial interfacial potential drops with ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ cathode, with (100) orientation showing a lower drop than (010). These insights enhance our understanding of the SCL's impact on Li-ion transport during cycling and suggest strategies for improving ASSBs' power output by tuning crystal orientation or the electrolyte valence band.