MgH2–TiH2 mixture as an anode for lithium-ion batteries: synergic enhancement of the conversion electrode electrochemical performance

Abstract

A 0.7MgH₂ + 0.3TiH₂ mixture was prepared by reactive grinding of Mg and Ti powders under hydrogen and tested as a conversion electrode for lithium-ion batteries. This composite presents superior electrochemical properties compared to MgH₂ or TiH₂ based electrodes, either in terms of reversible capacities or polarization versus applied current rate. A substantial reversible capacity of 1540 mA h g⁻¹ is measured at a suitable potential of 0.52 V vs. Li⁺/Li⁰ (current rate 0.1Li h⁻¹). For the same current rate, electrode polarization is limited to 0.2 V and stabilizes at 0.46 V for 1.0Li h⁻¹ while a continuous polarization increase is observed for MgH₂ based electrodes. In situ XRD analyses of this composite demonstrate that Mg and Ti hydrides are both electrochemically active and contribute together to the capacity. TiH₂ improves MgH₂ conversion process kinetics while MgH₂ enables a reversible conversion reaction for TiH₂. The three consecutive reactions are: (1) MgH₂ + 2Li⁺ + 2e⁻ ⇌ Mg + 2LiH; (2) δ-TiH₂ + xLi⁺ + xe⁻ ⇌ δ-TiH_2−x + xLiH (x ≤ 0.5) and (3) δ-TiH_2−x + (2 − x)Li⁺ + (2 − x)e⁻ ⇌ α-Ti + (2 − x)LiH (x = 0.5). This is, to our knowledge, the first example of a reversible lithium conversion process with TiH₂ powder.