Abstract
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. MgH2 constitutes one of the most attractive metal hydrides with a reversible capacity of 1480 mA·h·g−1 at 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 Mg2MHx and other Mg-based hydrides. The reversible conversion reaction mechanism of MgH2, which is lithium-controlled, can be extended to others hydrides as: MHx + 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 TiH2 and Mg2FeH6, Mg2CoH5 and 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 MgH2 particle 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.
Highlights
To satisfy the continuously raising need for energy is a key priority worldwide
The purpose of this review is to describe the properties of these metal hydrides properties in the reaction vs Li+/Li0 with a focus on thermodynamics, involved reaction mechanisms, and some key issues to improve the performance of hydride-based electrodes
Reaction of Mg2MHx with lithium After studying the reaction of titanium hydride with lithium, during which a reaction path involving the formation of the metastable fco δ-TiH phase occurs, the complex hydrides Mg2FeH6, Mg2CoH5, Mg2NiH4 were chosen as models system for a conversion process with high energy storage capacities and unusual thermodynamics properties [13,19]
Summary
To satisfy the continuously raising need for energy is a key priority worldwide. The challenge is to obtain environmentally friendly renewable power sources with enhanced electrical energy conversion efficiency at moderate costs. Compared to other conversion compounds MgH2 exhibits remarkable properties such as the lowest polarization value for conversion electrodes (less than 0.2 V) at an average potential of 0.5 V vs Li+/Li0 and a high reversible capacity (1480 mA·h·g−1 which is four times that of Li/C electrodes) All these properties make MgH2 suitable as a material for negative electrodes. MgH2 reacts with lithium ions in a reversible lithium-driven conversion reaction generating lithium hydride and magnesium metal: MgH2 + 2Li+ + 2e− Mg + 2LiH. This conversion reaction is not restricted to MgH2. The purpose of this review is to describe the properties of these metal hydrides properties in the reaction vs Li+/Li0 (conversion reaction) with a focus on thermodynamics, involved reaction mechanisms, and some key issues to improve the performance of hydride-based electrodes
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