Charge Storage Mechanism and Structural Evolution of Viologen Crystals as the Cathode of Lithium Batteries

Although organic ionic crystals represent an attractive class of active materials for rechargeable batteries owing to their high capacity and low solubility in electrolytes, they generally suffer from limited electronic conductivity and moderate voltage. Furthermore, the charge storage mechanism and structural evolution during the redox processes are still not clearly understood. Here we describe ethyl viologen iodide (EVI₂ ) and ethyl viologen diperchlorate (EV(ClO₄ )₂ ) as cathode materials of lithium batteries which crystallize in a monoclinic system with alternating organic EV²⁺ layers and inorganic I^- /ClO₄ ^- layers. The EVI₂ electrode exhibits a high initial discharge plateau of 3.7 V (vs. Li⁺ /Li) because of its anion storage ability. When I^- is replaced by ClO₄ ^- , the obtained EV(ClO₄ )₂ electrode displays excellent rate performance with a theoretical capacity of 78 % even at 5 C owing to the good electron conductivity of ClO₄ ^- layers. EVI₂ and EV(ClO₄ )₂ also show excellent cycling stability (capacity retention >96 % after 200 cycles).