The methods to measure solid electrolyte interphase (SET) electrochemical properties and SEI formation capability of non-aqueous electrolyte solutions are rarely addressed in the literature. And yet, there is a strong demand for new electrolyte generations that promote stabilized SEIs and have an influence to resolve safety, calendar life and other limitations of Li-ion batteries. To fill this gap, an in situ electrochemical approach with new descriptive criteria for highly quantitative characterization of SET and electrolytes is proposed. These criteria are: SET formation capacity, SET corrosion rate, SEI maintenance rate, and SET kinetic stability. These criteria are associated with battery parameters like irreversible capacity, self-discharge, shelf-life, power, etc. Therefore, they are especially useful for electrolyte development and standard fast screening, allowing a skillful approach to narrow down the search for the best electrolyte. The characterization protocol also allows retrieving information on interfacial resistance for SET layers and the electrochemical window of electrolytes, the other important metrics of characterization. The method validation was done on electrolyte blends 1.2M LiPF6 EC+EMC + X (16:64:20) (v/v%); where EC is ethylene carbonate, EMC is ethyl methyl carbonate and X is phosphazene. Several phosphazene varieties were developed with a target for safer electrolyte formulations. (C) 2014 The Electrochemical Society. All rights reserved.