Trifunctional Electrolyte Additive Hexadecyltrioctylammonium Iodide for Lithium–Sulfur Batteries with Extended Cycle Life

Lithium-sulfur (Li-S) battery with a very high theoretical energy density (∼2500 Wh kg^-1) is a very promising alternative to the commercial lithium-ion battery as the next-generation energy storage device. However, the Li-S battery suffers from shuttle effect and Li dendrites growth due to the solubility of polysulfides in the electrolyte system and the inhomogeneous deposition of Li, resulting in short cycling life span, which is the major obstacle in its practical application. Herein, we report an additive, hexadecyltrioctylammonium iodide (HTOA-I), in the conventional electrolyte system, which shows trifunctional effect on extending Li-S battery cycle life. It can not only help us to form a protective solid-electrolyte interface (SEI) on the surface of Li anode so as to reduce the contact of polysulfides with Li but also hinder the shuttling of polysulfides to the Li anode due to the strong combination of large-sized HTOA⁺ with polysulfide anions (S_n^2-), which retard the migration of S_n^2- and cause homogeneous Li deposition owing to the large size and stronger trend of HTOA⁺ to be absorbed on Li anode as well. A new method of phosphorescence analysis for direct observation of polysulfides shuttling has been put forward for the first time, which can be further developed in future studies. The cell with the HTOA-I-added electrolyte system shows high cycling stability, retaining 83.4% of the initial capacity after 200 cycles at 1 A g^-1 and achieving 689 mAh g^-1 even after 1000 cycles. This cost-effective and facile approach will not increase the complexity of the battery manufacturing process. Compared to other electrolyte additives, the additive in our work, HTOA-I, has better positive effects on extending cycle life. This trifunctional electrolyte additive will inspire the design of other new additives and further promote the development of Li-S batteries.