High-entropy oxychloride increasing the stability of Li–sulfur batteries

Abstract

A novel lithiated high-entropy oxychloride Li_0.5(Zn_0.25Mg_0.25Co_0.25Cu_0.25)_0.5Fe₂O_3.5Cl_0.5 (LiHEOFeCl) with spinel structure belonging to the cubic Fdm space group is synthesized by a mechanochemical–thermal route. Cyclic voltammetry measurement of the pristine LiHEOFeCl sample confirms its excellent electrochemical stability and the initial charge capacity of 648 mA h g⁻¹. The reduction of LiHEOFeCl starts at ca. 1.5 V vs. Li⁺/Li, which is outside the electrochemical window of the Li–S batteries (1.7/2.9 V). The addition of the LiHEOFeCl material to the composite of carbon with sulfur results in improved long-term electrochemical cycling stability and increased charge capacity of this cathode material in Li–S batteries. The carbon/LiHEOFeCl/sulfur cathode provides a charge capacity of 530 mA h g⁻¹ after 100 galvanostatic cycles, which represents ca. 33% increase as compared to the charge capacity of the blank carbon/sulfur composite cathode after 100 cycles. This considerable effect of the LiHEOFeCl material is assigned to its excellent structural and electrochemical stability within the potential window of 1.7 V/2.9 V vs. Li⁺/Li. In this potential region, our LiHEOFeCl has no inherent electrochemical activity. Hence, it acts solely as an electrocatalyst accelerating the redox reactions of polysulfides. This can be beneficial for the performance of Li–S batteries, as evidenced by reference experiments with TiO₂ (P90).