1. Battery energy storage systems: A methodical enabler of reliable power 2. Separators: an essential barrier between electrodes Part I: Separators for Non-Aqueous Batteries 3. Introduction to Separators for Non-Aqueous Batteries 4. Separators for Lithium Ion Batteries 5. Advanced Separators for Lithium Sulfur Batteries 6. Lithium Ion Conducting Membranes in Lithium-Air Batteries 7. Designing of Ion Transport Pathways in Separator for Lithium Ion Batteries Part II: Separators for Aqueous Batteries 8. Introduction to Separators for Aqueous Batteries 9. Lead Acid Battery Separators 10. Alkaline Zn-MnO2 Battery Separators 11. Redox Flow Batteries Part III: Theoretical Predictions and Future Challenges 12. Theoretical Simulations of Lithium-Ion Batteries 13. New Challenges of Battery Separators