Dimolybdenum paddlewheel complexes with cation binding sites as electrolyte additives to manipulate the solid-electrolyte interphase at lithium metal anodes

Use of electrolyte additives at mM loadings to control the surface chemistry of lithium metal anodes (LMAs) is a leading strategy to improve safety, efficiency, and reliability of lithium metal batteries and promote various types of electrosynthetic reactions. Whereas previous studies employed either inorganic or organic additives, in this study we report the first organometallic additive, Mo2(mea)4 [1, mea = 2-(2- methoxyethoxy)acetate], a dimolybdenum paddlewheel complex that is stable under Li plating conditions. Unlike more classically used crown ether additives, complex 1 features cation binding sites in the second coordination sphere that promote reversible Li+ coordination. Furthermore, binding of Li+ ions to 1 induces aggregation of cationically charged coordination oligomers/polymers that assemble at the LMA surface due to electrostatic attraction. Upon surface immobilization and incorporation into the solid- electrolyte interphase (SEI), the additive serves multiple beneficial functions. It was found to protect the LMA against parasitic side reactions by chemical passivation, produce modest but measurable improvements to Li plating properties (e.g. overpotential, surface structure, Coulombic efficiency), and modulate the composition and thickness of the SEI. The latter effect translates to significantly decreased resistance in battery settings due to improved interfacial charge transport properties. The most notable benefit to battery cycling performance comes from calendar aging tests, which show that the presence of the additive protects the LMA from parasitic side reactions that would otherwise decrease overall cell cycling efficiency Collectively, these data disclose a new tactic for designing electrolyte additives using principles of organometallic synthesis.