Ab Initio Molecular Dynamics of Li-Metal Anode in a Phosphate-Based Electrolyte: Solid Electrolyte Interphase Evolution

An interfacial study is performed using ab initio molecular dynamics simulations to elucidate the solid-electrolyte interphase (SEI) evolution between an electrolyte, based on trimethyl phosphate (TMP) and lithium bis(fluorosulfonyl)imide (Li+FSI−), and a Li-metal electrode (anode). We found a variety of products due to FSI-counterion dissociation such as Li2O, Li2S, Li3N and LiF, followed by Li2S, Li2O, LiF, Li3NSO2, during the second FSI dissociation, and Li2SO2NSO2 and LiF during the third one; then, the SEI prevents complete dissociation of the FSI as more counter-ions arrive to the interface. We report the electrochemical window stability to identify the energy barriers the electron must overcome to be transferred from the anode to the counter-ions to initiate the FSI dissociation. Using a nanobattery as a symmetrical cell, The anode open-circuit potential increases from −0.39, when there is not SEI, to 1.3 V when there is a SEI formed (in the anode) due to FSI dissociations. A method is presented to scale-up the nano-battery results into a real macro-battery; we identify the number of counter-ions that need to be consumed to obtain a stable SEI in a CR2032 coin battery and a linear extrapolation is performed to scale the SEI thickness as the counterions dissociate.