Dimerizing Lawsone into Bis-lawsone to Counter Solubility and Attain Facile Zn2+ Ion Diffusion for Stable Capacity in Aqueous Zinc-Ion Batteries

Abstract

Given the higher volumetric capacity of Zn (5853 mAh cm−3), aqueous zinc-ion batteries can be considered as alternatives to lithium-ion batteries for energy storage applications. Here we focus on redox-active, non-toxic, and eco-friendly organic materials with quinone moiety, i.e., Lawsone (LS) and its dimer Bis-lawsone (BL), as a cathode material for Zn-ion batteries. The reversibility of LS and BL is confirmed by cyclic voltammetry between 0.3 and 1.6 V in zinc sulfate and zinc triflate electrolytes. The electrochemical performance of BL in the zinc triflate electrolyte is improved by the reduction in the solubility of BL in the electrolyte and improvement in Zn2+ diffusion in the BL matrix. Besides, it is known that Zn electrodes passivate better in zinc triflates, which contributes to improved plating/stripping of Zn. The dimerization strategy counteracts solubility and facilitates the diffusion of Zn2+, resulting in a stable charge–discharge with a specific capacity of 200 mAh g–1. The cell attained 77% of the theoretical capacity at a current density of 100 mA g–1. Zn-BL cells show fast kinetics and reversibility for up to 760 cycles retaining 85% of initial capacity despite the partial solubility of BL in electrolytes. In addition, the Nafion-212 membrane is introduced as a separator instead of a traditional glass fiber separator. The negatively charged backbone of Nafion helps eliminate the crossover problem by coulostatic repulsion with negatively charged BL and LS.