Na-ion batteries are standing as a serious contender to the Li-ion technology for mass storage applications provided we fully master their chemistry, among which the electrolyte is of paramount importance. It controls the degree of parasitic reaction that results in the growth of the solid electrolyte interface (SEI) which governs the battery performances in terms of capacity retention, lifetime, etc… Herein, we show how cyclic voltammetry (CV) can be used to rapidly spot hints of electrolyte decomposition and determine whether the resulting species are either solubilized or adsorbed leading to the SEI formation. Using this approach, we identified a new electrolyte, which consists of a solution of 1M NaPF 6 in EC-DMC (1:1 v/v ratio) to which we added three additives namely vinylene carbonate (VC), sodium (oxalate) difluoro borate (NaODFB) and tris (trimethylsilyl) phosphite TMSPi. This novel electrolyte when implemented in today's practical full Na 3 V 2 (PO 4) 2 F 3 /C Na-ion cells gives the best high temperature performances in terms of cyclability and self-discharge. Using CV we could rationalize this finding and unambiguously prove that NaODFB is ruling the SEI formation while TMSPi is essential to control its growth and for capturing both O 2 and acid impurities responsible for deleterious reactions occurring at relatively high potentials.