Organic Cathodes for Energy Storage Systems

Abstract

Redox-active organic molecules have the potential as economical and environmentally benign cathodes for lithium ion batteries. The same concept of using organic molecules as cathodes was extended to magnesium ion batteries, however, pervious work showed considerable capacity loss upon the cycling. Furthermore, no compatible electrolytes toward both anode and cathode have been demonstrated. Her, we have focused on several organic cathode materials as cathodes, mainly for magnesium ion batteries. For example, 2,5-dimethoxy-1,4-benzoquinone (DMBQ) was reinvestigated as a cathode material with magnesium electrolytes that are capable of plating/stripping magnesium for rechargeable magnesium-ion batteries. Two electrolytes, the magnesium bis(trifluoromethylsulfonyl)imide mixed with MgCl2 in dimethoxyethane (Mg(TFSI)2-2MgCl2 in DME) electrolyte, and the Mg(TFSI)2 in diglyme were selected. The Mg(TFSI)2-2MgCl2 in DME enabled Mg-DMBQ batteries with a discharge potentials above 2.0 V vs Mg/Mg2+, which is superior to the previous reported potential in Mg-DMBQ batteries with conventional magnesium salt-based electrolytes (1.1 V vs Mg/Mg2+ ), and also excels the well-known Chevrel phase Mo6S8 in magnesium-ion batteries (1.2 V vs Mg/Mg2+). The other organic cathodes explored for magnesium ion batteries are the redox active polymers. With emphasis on the structure motif, several polymers were investigated as high performance cathode candidates for magnesium storage. Excellent electrochemical reversibility was established for polymers within the range of 1.5-2.0 V (vs Mg), with a 99% capacity retention in 100 cycles (104.9 mAh g-1 at 100th cycle). The superior cycling stability was further demonstrated by 1000 discharge-charge cycles with very small amount of capacity loss, highlighting the great potential of redox-active organic polymer cathodes for advanced Mg-ion batteries. Figure 1