High capacity and high voltage cathodic materials are key to increase the power and energy densities of rechargeable batteries.  The cause of instability of the high voltage cathodic materials is that these cathodic materials irreversibly react with the electrolyte solutions during charging/discharging and/or storage processes.  As a result of such reactions, the cathodic materials decompose to non-electrochemical active materials, and the same time electrolyte solution decomposes to gaseous molecules, leading to the batteries dry out condition, a deadly condition for rechargeable batteries.  Fundamentally, there are approaches to solve this problem as demonstrated in the literature: 1) preparation of new electrolytes and solvents that are thermodynamically stable, i.e. their first oxidation and first reduction potential are outside the potential of anodic redox couple (e.g. Li⁺/Li) and the potential of the cathodic redox couple (e.g. Li_(1-x)CoPO₄/LiCoPO₄); 2) discover new electrolytes and solvents systems that are kinetically stable, i.e. the electron transfer rates between electrolyte solution and both cathodic and anodic materials are slow, providing large overpotentials for those unwanted electrochemical reactions; 3) addition of new reagents (e.g. additives) into the electrolyte solutions to form an interfacial layer (e.g. SEI layer) on both cathodic and anodic materials, resulting slow electron transfer rate between electrolyte solution and both cathodic and anodic materials, providing, again, large overpotentials for those unwanted electrochemical reactions.  

               Many works have done over the past decade or so to improve the stability of the cathodic materials.  Due to synthetic challenges possessed in preparation of thermodynamically stable electrolyte solutions, discovering new additives that can slow down the ET processes becomes another popular approach.  Here we report the use of new polyfluorinated additives that can improve the stability of high voltage cathodic materials (e.g. LiCoPO₄) by slowing down the ET process between electrolyte solution and the cathodic materials during the charging/discharging processes by forming a fluorinated protective SEI layer.  The high electronegativity of fluorine results in exceptionally strong bonds to carbon (bond dissociation energy (BDE) C-F = 110-126 kcal/mol). Furthermore, fluorine substitution results in an increase in the C-C bond energy in fluorocarbons compared to the corresponding hydrocarbons.  The preformed fluorinated protective SEI layer on cathodic material surface either prevents or slows down the ET process between commonly used electrolyte solutions (i.e. PC with 1.0 M LiPF₆).  We find that such protective SEI layer can be pre-formed when treating the cathodic materials with polyfluorinated additives.  This result indicates that pre-modification of cathodic materials is possible to increase the stability and compatibility with the electrolyte solution.