Scale matters: simulation of nanoscopic dendrite initiation in the lithium solid electrolyte interphase using a machine learning potential

Although lithium solid state electrolytes promise to mitigate the chemical instabilities of liquid electrolytes in today's mainstream rechargeable batteries, solid state electrolytes still suffer from dendrite formation which leads to battery degradation and short circuiting. Dendrite initiation and propagation in specific solid state electrolyte materials has been explained, at a microscopic scale, as emerging from the lithium-filling of pores within the solid state electrolytes via microcracks. At the atomistic scale, the thermodynamic instability of many solid state electrolyte materials can explain their susceptibility to crystal decomposition upon contact with the lithium anode. However, for a more complete picture of the dendrite formation mechanisms, an understanding of the dendrite initiation mechanism at the intermediate nanoscopic scale is required. This work applies a machine learning potential (DIEP) for simulating six different solid state electrolyte-lithium interfaces at 300 K and 1000 K, with model sizes ranging from 24k to 36k atoms, for durations exceeding 20 ps. Our simulations show that the lithium dendrite initiation process can have an underpinning nanoscopic mechanism, in which the crystal decomposition by direct lithium interaction leads to the clustering of lithium. The simulations also suggest a possible mechanism for the creation of voids within the solid-electrolyte interphase, which have been observed in the Li$|$Li$_6$PS$_5$Cl$|$Li interface.