Electrolyte as the most flammable component of lithium ion battery is always considered to be closely related to its safety. Great efforts are made to optimize electrolyte since it is the ultimate means to improve the lithium ion battery safety. This article reviews the thermal risk of commercial electrolytes and the development of safer electrolytes. The main reason for the thermal instability of the traditional nonaqueous electrolyte is the thermal decomposition of lithium hexafluorophosphate (LiPF6) and highly flammable solvents. Substitution technique of the lithium salt is under developing and the electrolyte flame retardant additives are widely studied. Novel addition technologies like electrospinning and microcapsules are introduced to reduce the restrictions on physical properties of flame retardants and improve electrochemical performances. Overcharge protection additives are simply summarized according to their reaction mechanism. For the breakthrough of new generations of safer electrolytes, nonflammable solvents with new salts and solid state electrolytes are reviewed as well as their existing problems at present. This shall serve as a summary for the development of electrolytes and a reference for the design of next generation of safer electrolytes.