Long Cycle Life Organic Polysulfide Catholyte for Rechargeable Lithium Batteries.

Organic compounds with active sites for lithiation can be used as electrode materials for lithium batteries. Their tunable structures allow a variety of materials to be made and investigated. Herein, a spectrum of dipyridyl polysulfides (PyS , 3 ≤ ≤ 8) is prepared in electrolyte by a one-pot synthesis method from dipyridyl disulfide (PyS) and elemental sulfur. It renders up to seven dipyridyl polysulfides (i.e., PyS, PyS, PyS, PyS, PyS, and PyS) which show fully reversible electrochemical behavior in lithium batteries. In the discharge, the initial lithiation occurs at 2.45 V leading to the breakage of S-S bonds in PyS and formation of lithium 2-pyridinethiolate, in which lithium is coordinated in between N and S atoms. The left sulfur species act as elemental sulfur, showing two voltage plateaus at 2.3 and 2.1 V. The molecular dynamics simulations show the attraction between pyridyl groups and lithium polysulfides/sulfide via N···Li···S bonds, which enable good retention of soluble discharge products within electrodes and stable cycling performance. In the recharge, low-order PyS (e.g., PyS, PyS, and PyS) remain as the charged products. The mixture catholyte exhibits superlong cycle life at 1C rate with 1200 cycles and 70.5% capacity retention.