This research paper discusses the fundamental importance of energy storage technologies in mitigating the intermittency and variability of renewable energy sources such as solar, wind, hydropower, tidal and wave energy, geothermal energy, and biomass. Energy storage enables reliable grid integration, reduces dependence on fossil fuels, and supports global decarbonization efforts. The review explores mechanical storage systems (pumped hydro, flywheels, compressed air, and gravity-based systems), outlining their operating principles, advantages, limitations, and applications. A major focus is placed on electrochemical storage technologies, highlighting material innovations in batteries and supercapacitors — including lithium-ion chemistries (NMC, LFP, LTO), flow batteries, sodium-sulfur, and sodium-ion systems. Emerging developments such as hybrid battery-supercapacitor systems are presented as efficient solutions for grid stability and energy density optimization. The paper concludes by emphasizing future research directions in sustainable materials, hybrid configurations, and policy frameworks to accelerate the transition toward a carbon-neutral energy future.