A composition-of-matter comprising a plurality of particles is disclosed herein, the particles comprising a compound (e.g., an element or a mixture of elements) which forms an alloy with an alkali metal and/or an alloy of an alkali metal with said compound. The alloy is characterized by reversibly releasing the alkali metal and absorbing the alkali metal. Some or all of the particles are encapsulated within a volume enclosed by a shell or matrix which conducts cations of the alkali metal, wherein a volume of the alloy upon maximal absorption of the alkali metal does not exceed the volume enclosed by a shell or matrix. Further disclosed herein is a process of preparing a composition-of-matter, which is effected by coating particles comprising an alloy saturated with the alkali metal with a conductor of cations of the alkali metal, as well as electrochemical half cells and batteries including the composition-of-matter.