Insight into the Li- and Zn-Ion Synergistic Effect for Benzoquinone-Based Anodes in Aqueous Batteries

Aqueous rechargeable batteries are promising candidates for safe and large-scale energy storage systems, but their electrochemical performances are limited by anode degradation. Herein, poly(2,5-dihydroxy-1,4-benzoquinone-3,6-methylene) (PDBM) is introduced as a typical benzoquinone-based anode material in an aqueous rechargeable battery. A superior higher specific capacity and longer cycling life (255 mA h g(-1) and 83% capacity retention for 700 cycles at 1 C) were obtained in a mixed 1 M Li2SO4//2 M ZnSO4 electrolyte, which is better than that in a 1 M Li2SO4 (150 mA h and 76% for 700 cycles) or in a 2 M ZnSO4 electrolyte (206 mA h g(-1) and 76% for 700 cycles). A defined synergistic mechanism of Li+ and Zn2+ during redox reactions is presented, in which Zn2+ ions previously occupied the out-of-plane sites with respect to the benzene rings so that the Li+ ions occupied the in-plane sites in subsequent reducing reactions. Such synergistic bonding of Li+ and Zn2+ with the polymer anode was first confirmed by density functional theory calculations and was defined as an "ion dense stacking effect". Then, a full battery was constructed using the PDBM anode and LiMn2O4 cathode, where an average voltage of 1.15 V, an ultrahigh capacity of 228 mA h (vs anode), and a long lifespan of over 700 cycles (i.e., capacity retention, 81% at 1 C) were obtained. This is a pioneering study toward designing large-scale energy storage with a low-cost, high energy density, and ultralong life.