This chapter discusses sodium-ion batteries (SIBs), a cost-effective, sustainable alternative to lithium-ion batteries, leveraging abundant sodium resources. It covers their operational mechanism, where sodium ions shuttle between positive (e.g., layered oxides, polyanionic compounds, Prussian blue analogs) and negative electrodes (e.g., hard carbon, titanium-based, alloy-based materials). Key electrochemical properties, including voltage, capacity, and cycle life, are detailed, alongside advancements in electrolytes and separators to enhance performance and safety. Safety strategies addressing thermal stability and regulatory needs are discussed. The chapter highlights SIBs’ potential for grid storage and low-speed vehicles, driven by ongoing material and safety innovations.