In this work a combination method of A. C. Impedance and chronoamperometry was used to determine the lithium ion transport number of LiClO_4·(PEO)_(20)·(PC)_(12)·(EC)_(12) polymer electrolyte. It is known that lithium is thermodynamically unstable in non-aqueous or polymer electrolytes, leading to the formtion of a film on lithium surface, called as solid electrolyte interphase (SEI). The presence of SEI layer will greatly influence the experimental result of lithium ion transport number. The new method avo...