A brief review of our fundamental studies of the reversible reactivity mechanism of vanadates towards lithium is presented. This mechanism totally differs from the classical one based either on reversible insertion/deinsertion of lithium into host structures or on Li alloying reactions, and has led to the thought of using nanosized transition metal oxides as possible negative electrode materials for rechargeable Li-ion batteries. Electrochemical capacities, as high as 700 mAh/g with a 100% capacity retention up to 100 cycles and high rates, can be achieved with optimized metal oxides (MO, M = Co, Cu, Ni, Fe, etc.) powders. By combining transmission electron microscope (TEM), infrared (IR) and magnetic measurements we directly proved the formation of 10-50 Angstrom metal nanoparticles dispersed into a lithia (Li2O) matrix during the reduction step of MO with Li. Upon oxidation, the metal nanoparticles were shown to convert back to MO while Li2O was decomposed. The new opportunities provided by these metal oxides systems based on the reversible formation/decomposition of Li2O are discussed together with the positive attributes that nanoparticles could have to the field of energy storage.