Mechanochemical synthesis of sustainable energy materials

The synthesis of new materials for energy storage and conversion has received increasing attention over the last decade. This growing importance reflects the burgeoning energy demand of an escalating world population, the finite supply of fossil fuels and the urgent need to reduce carbon dioxide emissions. Synthetic methods in which reactions are driven mechanically (effectively by the transfer and conversion of kinetic energy) can be considered as a branch of green chemistry. Materials are impacted by pressures that are created between the grinding media and with the container walls through multiple collisions during the milling process. These events induce mechanical deformation and define the nature of the milled products. This article focuses on the application of physical and – principally – chemical (reactive) milling in the preparation of energy-related materials. For hydrogen storage, complex magnesium hydrides prepared by reactive milling exhibit lower desorption temperatures and faster kinetics than those of magnesium hydride itself. The complete conversion of lithium nitride to lithium amide, the thermodynamically unfavorable hydrogenation of magnesium nitride and the synthesis of metal amides from metal hydrides can all be conducted in the mill. In the context of energy storage materials, nanostructuring is also one method by which the volume expansion associated with lithiation in high-capacity alloy anodes in lithium ion batteries can be mitigated. Mechanochemical synthesis provides one method for achieving such nanostructured materials and offers, for example, a unique approach in the synthesis of tin nanowires for use as negative electrodes in secondary lithium ion batteries.