Aluminum intercalation behaviours of {[Fe(Tp)(CN)3]2[M(H2O)2]} cyanido-bridged chain compounds in an ionic liquid electrolyte

Abstract

As the development of aluminum-ion batteries is still in its infancy, researchers are still dedicated to exploring suitable host materials and investigating their aluminum intercalation behaviours. Here, a series of cyanido-bridged chain compounds with the formula {[Fe^III(Tp)(CN)₃]₂[M^II(H₂O)₂]}_n (M = Ni, Co, Mn, Zn, Cu) are studied as cathode electrodes for aluminum-ion batteries with [EMIm]Cl–AlCl₃ (1-ethyl-3-methylimidazolium chloride–AlCl₃) ionic liquid as the electrolyte. The electrochemical properties suggested Fe³⁺/Fe²⁺ to be the redox-active couple during the aluminum intercalation and deintercalation processes of these compounds, and the observed maximum specific capacity obtained by the Fe–Co compound is 200 mA h g⁻¹ despite the rapid specific capacity fading. To gain a deeper understanding of the capacity decay suffered by these compounds, further investigation was conducted to explore the evolution of compounds during the electrochemical measurements. It has been attributed to the following reasons: 1. thermodynamic instability results in the transformation/damage of two of the chain structures (for the Fe–Ni and Fe–Co compounds) during heat treatment on electrodes, a crucial step in electrode preparation; 2. the acidic nature of the electrolyte triggers the destruction of the chain structure, with the appearance of partial reduction of Fe³⁺ to Fe²⁺, and a new interaction of the cyano group with aluminum; 3. the high charge density of inserted Al ions makes the chain structure suffer from structural damage during both the charging and discharging processes. The progressive accumulation of trapped intercalated ions hampers their involvement in the reaction, consequently decreasing electrochemical reversibility.