Hierarchical Carbon Nanofiber/Birnessite-MnO2 Nanosheets Cathode for High Performance and Low-Cost Aqueous Rechargeable Zinc Ion Battery

Abstract

Abundant and safe charge carrying feedstock with high energy density have made aqueous zinc-ion batteries (ZIBs), specifically the Zn/MnO2 system, a highly promising candidate for large-scale energy storage device. The traditional alkaline Zn/MnO2 batteries have become dominant in primary battery chemistry. Great efforts have been devoted to developing rechargeable alkaline Zn/MnO2 batteries, however, they are plagued by poor cycling stability. Very recently, the Zn/MnO2 systems in near neutral electrolyte have emerged to demonstrate improved capacities, cyclability and rechargeability. However, the commonly used tunnel-type MnO2 cathode with low conductivity usually undergoes phase transition to layered structure upon discharge-charge cycles, leading to collapse of framework and thus poor cycling and rate performance. To address these issues, it is proposed that directly using birnessite-MnO2 with layered structure as a cathode may minimize the “sacrificial capacity” induced by phase transition and integrating birnessite-MnO2 with a conductive substrate may enhance the conductivity. In this work, the hierarchical core-shell carbon nanofiber@MnO2 (CNF@MnO2) cathode was prepared by a facile wet-chemistry method, where vertically aligned birnessite-MnO2 nanosheets were in situ grown on the surface of carbon nanofibers. The self-supported CNF@MnO2 cathode showed excellent electrochemical performance for Zn-ion batteries in a near neutral electrolyte due to its structural stability and enhanced conductivity. A high discharge capacity of 306 mA h g−1 has been reached after 100 cycles at current density of 300 mA g−1. Even at a high rate of 1.5 A g−1, CNF@MnO2 still maintained a high capacity of 258 mA h g-1 upon 100 cycles. Such remarkable performance is attributed to the unique core-shell structure and intimate contact between CNF and layered MnO2, in which the conductive CNF core facilitates the electron transfer and layered MnO2 minimizes the structural collapse and promotes the kinetics for co-insertion and extraction of Zn2+/H+ ions. Given the simplicity of preparation, exceptional electrochemical performance and low cost, CNF@MnO2 cathode holds substantial promise for aqueous rechargeable Zn-ion batteries. Figure 1. Scanning electron microscopy image of carbon nanofiber@MnO2 (CNF@MnO2) cathode for aqueous rechargeable Zn-ion batteries in near neutral electrolyte. Figure 1