Self-Assembled Macrocyclic Copper Complex Enables Homogeneous Catalysis for High-Loading Lithium-Sulfur Batteries.

Abstract

The practical viability of high-energy-density lithium-sulfur (Li-S) batteries stipulates the use of a high-loading cathode and lean electrolyte. However, under such harsh conditions, the liquid-solid sulfur redox reaction is much retarded due to the poor sulfur and polysulfides utilization, leading to low capacity and fast fading. Herein, a self-assembled macrocyclic Cu(II) complex (CuL) is designed as an effective catalyst to homogenize and maximize the liquid-involving reaction. The Cu(II) ion coordinated with four N atoms features a planar <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msub><mi>d</mi> <msup><mi>sp</mi> <mn>2</mn></msup> </msub> <annotation>${\mathrm{d}}_{{\mathrm{sp}}^{2}}$</annotation></semantics> </math> hybridization, showing a strong bonding affinity toward lithium polysulfides (LiPSs) along the <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><msub><mi>d</mi> <msup><mi>z</mi> <mn>2</mn></msup> </msub> <annotation>${\mathrm{d}}_{{z}^{2}}$</annotation></semantics> </math> orbital via steric effects. Such a structure not only lowers the energy barrier of the liquid-solid conversion (Li₂ S₄ to Li₂ S₂ ) but also guides a 3D deposition of Li₂ S₂ /Li₂ S. As such, with a 1 wt% electrolyte additive of CuL, a high initial capacity of 925 mAh g^-1 and areal capacity of 9.62 mAh cm^-2 with a low decay of 0.3%/cycle can be achieved under a high sulfur loading of 10.4 mg cm^-2 and low electrolyte/sulfur ratio of 6 µL mg_s ^-1 . This work is expected to inspire the design of homogenous catalysts and accelerate the uptake of high-energy-density Li-S batteries.