Caffeine as an energy storage material for next-generation lithium batteries

Organic compounds are attracting attention as potential candidates for next-generation energy storage materials for rechargeable batteries. The utilization of redox centers in naturally occurring and human-edible organic compounds has excellent potential for designing sustainable and safe energy storage materials. However, an in-depth understanding of the energy storage mechanism is an essential prerequisite for its use and optimization as an electrode material. In this study, we applied caffeine as an electrode material in lithium batteries and revealed the energy storage mechanism for the first time. Two equivalents of electrons and lithium-ions participate in redox reactions during the charge-discharge process, providing a reversible capacity of 265 mAh g−1 in a voltage window of 1.5–4.3 V. Furthermore, it was discovered that a new redox reaction site can be controlled through molecular tuning by halogenation, and the newly created redox site shows that oxidation voltage increases in proportion to the electronegativity of halogen elements in the order of I, Br, Cl, and F. These results demonstrate the applicability of nature-derived organic materials as energy storage materials and a feasible strategy to design tunable redox-active sites and their redox potential through molecular-scale engineering.