Resolving Self-Discharge Mechanisms in Anode-less Lithium 1 Metal Batteries

ABSTRACT: Anode-less Li metal batteries suffer from rapid self-discharge after plating Li metal during charge. The speculated mechanism of self-discharge involves a combination of direct reactions between Li metal and electrolyte components as well as a Cu-Li galvanic pathway involving electrolyte reduction at the copper-electrolyte interface. However, direct separation of these pathways and quantification of their relative contributions as a function of the amount of plated lithium (plated Li capacity) in full cells has not been achieved. In this study, we combine 7Li operando NMR, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) imaging to quantify Li loss rates, track SEI impedance growth and visualize Li morphology evolution throughout self-discharge as a function of charge capacity. We find compelling evidence that self-discharge is initially dominated by rapid passivation of Li metal through direct Li-electrolyte reactions, with preferential loss of high surface area Li deposits observed within the first two hours after charge. The initial rate of self-discharge is strongly dependent on the extent of Li plating, while long-term self-discharge rates are found to be relatively insensitive to plated-Li capacity. This suggests that on longer timescales, self-discharge kinetics are governed by galvanic corrosion. Self-discharge rates are accurately captured by a two-component kinetic model: a stretched exponential term describes the distributed kinetics of direct Li-electrolyte reactions (reflecting the range in Li morphologies), and a compressed exponential term describes the galvanic pathway (reflecting nucleation-propagation of Cu SEI on freshly exposed Cu surfaces).