In Situ Two‐Step Activation Strategy Boosting Hierarchical Porous Carbon Cathode for an Aqueous Zn‐Based Hybrid Energy Storage Device with High Capacity and Ultra‐Long Cycling Life

Aqueous Zn-based hybrid energy storage devices (HESDs) exhibit great potential for large-scale energy storage applications for the merits of environmental friendliness, low redox potential, and high theoretical capacity of Zn anode. However, they are still subjected to low specific capacities since adsorption-type cathodes (i.e., activated carbon, hard carbon) have limited capability to accommodate active ions. Herein, a hierarchical porous activated carbon cathode (HPAC) is prepared via an in situ two-step activation strategy, different from the typical one-step/postmortem activation of fully carbonized precursors. The strategy endows the HPAC with a high specific surface area and a large mesoporous volume, and thus provides abundant active sites and fast kinetics for accommodating active ions. Consequently, pairing the HPAC with Zn anode yields an aqueous Zn-based HESD, which delivers a high specific capacity of 231 mAh g^-1 at 0.5 A g^-1 and excellent rate performance with a retained capacity of 119 mAh g^-1 at 20 A g^-1 , the best result among previously reported lithium-free HESDs based on carbon cathodes. Further, the aqueous Zn-based HESD shows ultra-long cycling stability with a capacity retention of ≈70% after 18 000 cycles at 10 A g^-1 , indicating great potential for environmentally friendly, low-cost, and high-safety energy storage applications.