In order for battery manufacturers to have a sustainable business, the batteries they produce must be as safe as possible. For lithium-ion batteries, reducing the flammability of the electrolyte is considered to be one way to improve safety, which might be achieved by adding flame retardants to the electrolyte. On the other hand, sodium-ion batteries are attracting attention from academic researchers due to the abundance of sodium reserves compared to lithium reserves. However, there are virtually no studies about the safety of sodium-ion batteries. In this thesis, studies of these two issues will be reported. The reactivity of charged/discharged electrode materials for sodium-ion batteries in different solvents and electrolytes at elevated temperature was studied using Accelerating Rate Calorimetry (ARC). Hard carbon was studied as a negative electrode material for sodium-ion batteries. The reactivity of sodium-inserted hard carbon in solvents and electrolytes was investigated. Then, the reactivity of sodium-inserted hard carbon was compared to lithiated graphite. NaCrO2, NaxCoO2 and NaNi0.5Mn0.5O2 were studied as positive electrode materials for sodium-ion batteries. The electrochemical performance of these materials was investigated. The reactivity of charged NaCrO2, NaxCoO2 and NaNi0.5Mn0.5O2 in solvents and electrolytes was studied using ARC. Sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) was studied as an electrolyte salt for sodium-ion batteries. The electrochemical performance of hard carbon and NaCrO2 in NaTFSI/PC electrolyte was studied. The reactivity of sodium-inserted hard carbon and deintercalated NaCrO2 in NaTFSI/PC electrolyte was also investigated. Triphenyl phosphate (TPP) was studied as a flame retardant additive for lithium-ion batteries. Its impact on electrochemical performance of negative electrode materials (petroleum coke and graphite) and positive electrode materials (LiNi1/3Mn1/3Co1/3O2 (NMC) and LiNi0.8Co0.15Al0.05O2 (NCA)) was studied using an automated storage test, symmetric cells and Electrochemical Impedance Spectroscopy (EIS). The reactivity of lithiated graphite, deintercalated NMC and NCA in electrolyte containing TPP was investigated using ARC. Finally, the flammability of electrolytes containing TPP was studied using a Self-Extinguishing Time (SET) test.