A reversible self-assembled molecular layer for lithium metal batteries with high energy/power densities at ultra-low temperatures

Abstract

Electrolytes for low temperature, high energy lithium metal batteries are expected to possess both fast Li⁺ transfer in the bulk electrolytes (low bulk resistance) and a fast Li⁺ de-solvation process at the electrode/electrolyte interface (low interfacial resistance). However, the nature of the solvent determines that the two always stand at either ends of the balance, and conventional electrolyte designs have to make a compromise to favor one at the expense of the other. Here, we address this irreconcilable dilemma using an “electric-field assisted self-assembly layer”, i.e. sodium perfluorooctanoate (NaPFO). Driven by the bias-potential, PFO⁻ anions dissolved in the electrolyte could reversibly self-assemble into a dense and ordered molecular layer at the cathode/electrolyte interface, which could protect the electrolyte from anodic degradation due to the high voltage and enable the stable cycling of LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) in ether-based electrolyte with an ultra-low freeing point. More importantly, the self-assembly of the PFO⁻ layer could aid the Li⁺ de-solvation behavior at the cathode/electrolyte interface, thus significantly lowering the interfacial resistance. Thus, with this reversible self-assembly layer on the cathode surface, low bulk resistance and low interfacial resistance are simultaneously achieved. Consequently, the tetrahydrofuran (THF)-based electrolyte containing NaPFO is applied to practical Li‖NMC811 pouch cells and achieves an unprecedented energy density of 122 W h kg⁻¹ (except taps and packing foil, same hereafter) at −85 °C, the highest reported power density of 318 W kg⁻¹ at a high current density of 4.1 mA cm⁻² at −60 °C, and can be recharged with stable performance (175 W h kg⁻¹ at 20th cycles) at −40 °C. This electric-field assisted self-assembly layer enables fine tuning of the micro-environment at the cathode–electrolyte interface, and provides a new design concept for the electrolyte of ultra-low temperature high voltage lithium-metal batteries.