Extremely Pure Mg2FeH6 as a Negative Electrode for Lithium Batteries

Abstract

Nanocrystalline samples of Mg-Fe-H were synthesized by mixing of MgH2 and Fe in a 2:1 molar ratio by hand grinding (MIX) or by reactive ball milling (RBM) in a high-pressure vial. Hydrogenation procedures were performed at various temperatures in order to promote the full conversion to Mg2FeH6. Pure Mg2FeH6 was obtained only for the RBM material cycled at 485 °C. This extremely pure Mg2FeH6 sample was investigated as an anode for lithium batteries. The reversible electrochemical lithium incorporation and de-incorporation reactions were analyzed in view of thermodynamic evaluations, potentiodynamic cycling with galvanostatic acceleration (PCGA), and ex situ X-ray Diffraction (XRD) tests. The Mg2FeH6 phase underwent a conversion reaction; the Mg metal produced in this reaction was alloyed upon further reduction. The back conversion reaction in a lithium cell was here demonstrated for the first time in a stoichiometric extremely pure Mg2FeH6 phase: the reversibility of the overall conversion process was only partial with an overall coulombic yield of 17% under quasi-thermodynamic control. Ex situ XRD analysis highlighted that the material after a full discharge/charge in a lithium cell was strongly amorphized. Under galvanostatic cycling at C/20, C/5 and 1 C, the Mg2FeH6 electrodes were able to supply a reversible capacity with increasing coulombic efficiency and decreasing specific capacity as the current rate increased.