Construction of cobalt hexacyanoferrate multivoid nanoframes as efficient aqueous aluminum ion batteries

Abstract

In the search for innovative materials for aqueous Al-ion batteries, Prussian blue analogs (PBAs) have drawn significant attention due to their open-framework structures. Yet, PBAs face challenges like restricted capacity and rapid capacity deterioration, primarily stemming from the scarcity of redox sites and their inherent structural instability. In the current research, a synthesis of Fe–Co PBA multivoid nanoframe structures was achieved through straightforward co-precipitation techniques, aimed at optimizing Al-ion storage. Key to enhancing their performance was the modulation of the hydrothermal self-assembly reaction duration. As a result of this modulation, a morphological transition from nanocubes to multivoid nanoframes became evident, leading to the exposure of a higher number of active sites. This morphological transition also minimized the effects of volumetric shifts during the electrochemical charge/discharge cycles. Performance analysis indicated that the Fe–Co PBA multivoid nanoframes demonstrated a stable specific capacity, reaching 43 mAh g⁻¹ over 2000 cycles at 0.5 A g⁻¹ . Impressively, even under the more demanding current density of 1 A g⁻¹ , the specific capacity remained a substantial 36 mAh g⁻¹ after the same number of cycles. Comprehensive ex situ evaluations further underscored the predominant charge storage mechanisms, highlighting the reversible redox activities of the involved transition metals, coupled with the insertion/extraction dynamics of Al³ ⁺ ions.