A Li2FeSiO4 cathode material for lithium-ion batteries was synthesized by a molten carbonate flux method at 500 degrees C. The Li2FeSiO4 showed reversible capacity of 160 mAh g(-1) at the 0.07 C rate in the voltage range of 1.5 - 4.2 V. Crystal structural change during charge-discharge process for the Li2FeSiO4 was investigated using high-resolution synchrotron X-ray diffraction in order to understand charge-discharge performance. It was found that crystal structure of the Li2FeSiO4 transformed to a new structure of a charged phase and a discharged one during the first charge-discharge. Crystal structure of the charged phase and the discharged one was supposed to have a monoclinic system with space group P2(1)/n based on models of cations disordering of Li+, Fe2+ and Fe3+. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.jes112828] All rights reserved.