Successive Storage of Cations and Anions by Ligands of π-d-Conjugated Coordination Polymers Enabling Robust Sodium-Ion Batteries

The oxidation of π-d-conjugated coordination polymers (CCPs) accompanied with anion insertion has the merits of increasing the capacity and elevating the discharge voltages. However, previous reports on this mechanism either required more investigations or showed low capacity and poor cyclablity. Herein, triphenylene-catecholate-based two-dimensional CCPs are constructed by employing inactive transition-metal ions (Zn²⁺ ) as nodes, forming Zn-HHTP. Substantial characterizations and theoretical calculations indicate the successive storage of cations and anions by redox reactions of only ligands, leading to a high reversible capacity of ≈150 mAh g^-1 at 100 mA g^-1 and a remarkable capacity retention of 90 % after 1000 cycles. On the contrary, as a control experiment, the analogous CCPs (Cu-HHTP) with Cu²⁺ nodes, where both ligands and metal ions undergo redox reactions, accompanied by the storage of only Na⁺ cations, show a much poorer cyclability. These results highlight the importance of redox reactions of only ligands for long-term cycle life and the insight into the storage mechanisms deepens our understanding on CCPs for the further design of CCPs with high performance.