Born liquid to live solid: in situ polymerized electrolyte enables stable operation of organic-Li metal batteries

Abstract

We have designed an electrolyte composition capable of delayed self-polymerization by combining two commonly used electrolytes: 1 M LiTFSI in 1,3-dioxolane/1,2-dimethoxyethane and 1 M LiPF₆ in ethylene carbonate/dimethyl carbonate. It has been shown that LiPF₆ initiates self-polymerization of 1,3-dioxolane also involving dimethoxyethane molecules, which produces quasi solid (gel) electrolyte at the desired timescale after cell fabrication. This approach solves numerous technical issues such as poor adhesion of the solidified polymer electrolyte to the electrodes, incomplete soaking of the separator membrane, etc. Furthermore, we have serendipitously discovered that a new electrolyte formulation produces a specific solid electrolyte interface (SEI) on the lithium surface, which is beneficial for the battery operation. The advantages of the developed strategy have been demonstrated in batteries with an organic cathode and Li metal anode. While polythiopyranoquinone cathodes provided low capacities of ∼90 mA h g⁻¹ in LiTFSI and LiPF₆-based commercial liquid electrolytes when they were used separately, the engineered self-polymerizable electrolyte enabled a high reversible specific capacity of 342 mA h g⁻¹ with a capacity retention of 98.4% after 180 cycles. The observed improvement was due to the gelation of the electrolyte suppressing the dissolution of the organic electrode and also the formation of an optimal solid–electrolyte interface.