An ab initio molecular dynamics (AIMD) study of an electrochemical interface between a solid-state-electrolyte (SSE) Li9N2Cl3 and a Li-metal anode is performed to analyze the possibility of interphase formation due to electrochemical instability when the cell is at open circuit. Then an external electric field is applied to emulate the Li9N2Cl3/Li-metal interface under charging conditions. In the open circuit simulation, we observe a high electrochemical stability at both sides of the interface, without electrolyte decomposition or new phase formation at the interface; this is in good agreement with reported experimental data. However, a structural re-accommodation takes place at both sides of the interface. Some oxidation of the surface Li atoms takes place when Cl and N anions from the electrolyte capture Li atoms from the metal to complete their octahedral-like structures. At the interface, Li atoms surrounding Cl atoms show different behavior than those near N atoms. Once N captures a Li atom to complete the octahedral geometry observed in the Li9N2Cl3 bulk, this geometry becomes stable; when Cl is the capturing atom, Li is much mobile, thus, decreasing its Cl coordination number from the bulk value to 5. Bader charge and electron density analysis explain this difference between Cl and N behavior. At the interface of the Li9N2Cl3 (002) facet, the larger negative charge on low-coordinated N atoms is able to retain more Li atoms than the Cl atoms do. At charging conditions using an electric field of 0.5 V/Å, no major changes are observed in terms of atomic diffusion or structural changes at the interphase; however, the main difference is the change of the Li-metal crystal structure. Due to migration of Li from the metal anode to the electrolyte, vacancies are created on the Li-metal anode, causing a structural re-arrangement at the anode surface. Indeed, we anticipate that these interfacial migrations consume more Li from the metal anode, deforming the structure of additional inner Li (001) layers.

Solid-state electrolytes: Li-metal interfaces

2019 Fall National Meeting

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