Process for improving lithium ion cell

A process for increasing the thermal stability of a rechargeable lithium ion cell which is in a charged state wherein said lithium ion cell in said charged state has a negative electrode comprising carbon and lithium ions, a positive electrode comprising a lithiated metal oxide and an electrolyte comprising a metal salt in a non-aqueous solvent, and wherein said cell has a passivating layer on a surface of the carbon negative electrode;

• said process comprising the steps of maintaining said charged lithium ion cell at a temperature from about 45.degree. C. to about 60.degree. C. for a period of time of from about 1 hour to about 72 hours to increase the passivating efficacy of the carbon negative electrode passivating layer, said process having the effect of limiting exothermic reaction between electrolyte and said negative electrode upon exposure of said lithium ion cell to a temperature above about 100.degree. C.