Electrolyte comprising a phosphite as an additive or co-solvent, lithium rechargeable battery comprising said electrolyte, and method for producing the phosphite

A method of using an electrolyte in a rechargeable lithium battery at a voltage above 4.2 V, the method comprising:

• providing the electrolyte comprising,
• at least one lithium salt,
• at least one first organic solvent having a dielectric constant of >90 at 40° C.,
• at least one second organic solvent having a boiling point of <110° C., and
• at least one phosphite having the formula (I)
• wherein,
• R1 is a completely or partially halogen-substituted, linear or branched, C2-C20 alkyl group, and
• R2, R3, R4, and R5 are each selected, independently of one other, from H, or an unsubstituted or completely or partially halogen-substituted, linear or branched, C1-C20 alkyl group;
• providing the electrolyte as a part of the rechargeable lithium battery;
• and
• using the electrolyte in the rechargeable lithium battery at the voltage above 4.2 V.