Understanding the Effect of Pre-Intercalated Cations on Zn-Ion Storage Mechanism of Layered Birnessite Manganese Oxide for Aqueous Zn-ion Batteries

Abstract

With the rapid growth of energy consumption, tremendous research efforts have been dedicated to achieving sustainable and green energy storage systems, owing to environmental concerns. Over the past few years, rechargeable aqueous zinc-ion batteries (ZIBs) have become a compelling alternative to lithium-ion batteries (LIBs). Although LIBs have been successfully commercialized due to their high energy density, their organic-based electrolytes are highly volatile and flammable. Therefore, aqueous Zn-ion batteries have emerged as promising energy storage devices, owing to the benefits of water-based electrolytes such as low-cost, high safety, and high-power density as well as the advantages of zinc metal such as its natural abundance, low redox potential (-0.76 V vs standard hydrogen electrode), and high theoretical capacity (840 mAh g-1). Among cathode materials, birnessite manganese oxide (δ-MnO2) is one of the promising intercalation cathode materials for ZIBs due to its layered structure, which can facilitate H+/Zn2+ diffusion, leading to enhanced rate performance and improved capacity. However, the δ-MnO2 also suffers from some critical drawbacks such as structural phase transformations and collapse of the layered structure, resulting in poor cycling stability. To address these issues, a pre-intercalation strategy has been proved as an effective way to stabilize the layered MnO2 by using various organic polymers and metal cations such as polyaniline, Li+, K+, and Ca2+. In this work, we studied the effect of charge density of pre-intercalated cation on the Zn2+ storage mechanism of the δ-MnO2 in a mild aqueous electrolyte of 1 M ZnSO4. Overall, the results showed that a small amount of highly charged pre-intercalant (Al3+) can efficiently stabilize the layered structure of the δ-MnO2, resulting in more accommodation space for Zn2+ insertion, which can lead to improved capacity for the Al-MnO2 of 210 mAh g-1 at 0.1 A g-1. Apart from high capacity, the Al-MnO2 also exhibits excellent cycling stability with a capacity retention of 84% after 2000cycles at 2 A g-1. This is because highly charged intercalants can induce a reduction of binding energy between Zn2+ and host structure, leading to a highly reversible (de)intercalation of Zn2+ and stable structure of the δ-MnO2 during charge/discharge, which was confirmed by ex-situ XRD and DFT calculations. KEYWORDS: rechargeable aqueous Zn-ion batteries, birnessite manganese oxide, intercalation