Linear Polymer Cathode Materials for Highly Efficient Aqueous Zinc‐Ion Batteries: Is the High Active Sites Density Necessary?

Recently, research on aqueous zinc-ion batteries (AZIBs) has always focused on improving the energy storage performance by increasing the number of active sites, particularly in designing organic/polymer materials with a high density of active sites. However, does a higher density of active sites necessarily induce enhanced energy storage performance? To verify this issue, we have designed two linear polymers, where TAPT-DHBQ contains an additional pair of active sites (carbonyl groups) compared to TABQ-DHBQ, with theoretical specific capacities of 545.26 and 379.14 mAh g^-1, respectively. Interestingly, the experimental results have deviated with the specific capacities of these polymers being comparable, measuring to be 325 mAh g^-1 (TABQ-DHBQ) and 280 mAh g^-1 (TAPT-DHBQ). This is attributed to the competition effect between neighboring active sites, which leads to decreased utilization of active sites. As a result, the Zn//TABQ-DHBQ batteries with ZnI₂ electrolyte additive have exhibited high specific capacities of 618 and 360 mAh g^-1 at the current densities of 1 and 10 A g^-1, along with a high energy density of 678.6 Wh kg^-1 (1 A g^-1). The finding underscores the importance of uniform electron cloud distribution in cathode materials for achieving efficient AZIBs.