Climate change motivated by human activities constitutes one of the main challenges of this century. To cut carbon emissions to mitigate its dangerous effects, the current energy generation mix should be shifted to renewables sources. The main drawback of these technologies is their intermittency that will require storage systems to fully integrate them in the generation mix, allowing them to be more controllable. In recent years great progress to develop an effective and economically feasible energy storage systems, particularly motivated by the recent rise of demand for electric transportation has been made. Lithium-ion (Li-ion) battery prices have fallen near 90% over the past decade, making possible the affordability of electric vehicles and transforming the economics of renewable energy. In this paper, we evaluate the energy storage requirements for a carbon-free scenario in Spain, analyzing the CO2 emissions reduction.