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 (Py₂S , 3 ≤ x ≤ 8) is prepared in electrolyte by a one-pot synthesis method from dipyridyl disulfide (Py₂S₂) and elemental sulfur. It renders up to seven dipyridyl polysulfides (i.e., Py₂S₃, Py₂S₄, Py₂S₅, Py₂S₆, Py₂S₇, and Py₂S₈) 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_α-S_β bonds in Py₂S 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 Py₂S (e.g., Py₂S₃, Py₂S₄, and Py₂S₅) remain as the charged products. The mixture catholyte exhibits superlong cycle life at 1C rate with 1200 cycles and 70.5% capacity retention.