Metal hydrides: an innovative and challenging conversion reaction anode for lithium-ion batteries

The state of the art of conversion reactions of metal hydrides (MH) with lithium is presented and discussed in this review with regard to the use of these hydrides as anode materials for lithium-ion batteries. A focus on the gravimetric and volumetric storage capacities for different examples from binary, ternary and complex hydrides is presented, with a comparison between thermodynamic prediction and experimental results. MgH₂constitutes one of the most attractive metal hydrides with a reversible capacity of 1480 mA·h·g⁻¹at a suitable potential (0.5 V vs Li⁺/Li⁰) and the lowest electrode polarization (<0.2 V) for conversion materials. Conversion process reaction mechanisms with lithium are subsequently detailed for MgH₂, TiH₂, complex hydrides Mg₂MH_xand other Mg-based hydrides. The reversible conversion reaction mechanism of MgH₂, which is lithium-controlled, can be extended to others hydrides as: MH_x+xLi⁺+xe⁻in equilibrium with M +xLiH. Other reaction paths—involving solid solutions, metastable distorted phases, and phases with low hydrogen content—were recently reported for TiH₂and Mg₂FeH₆, Mg₂CoH₅and Mg₂NiH₄. The importance of fundamental aspects to overcome technological difficulties is discussed with a focus on conversion reaction limitations in the case of MgH₂. The influence of MgH₂particle size, mechanical grinding, hydrogen sorption cycles, grinding with carbon, reactive milling under hydrogen, and metal and catalyst addition to the MgH₂/carbon composite on kinetics improvement and reversibility is presented. Drastic technological improvement in order to the enhance conversion process efficiencies is needed for practical applications. The main goals are minimizing the impact of electrode volume variation during lithium extraction and overcoming the poor electronic conductivity of LiH. To use polymer binders to improve the cycle life of the hydride-based electrode and to synthesize nanoscale composite hydride can be helpful to address these drawbacks. The development of high-capacity hydride anodes should be inspired by the emergent nano-research prospects which share the knowledge of both hydrogen-storage and lithium-anode communities.