In situ X-ray diffraction and electrochemical impedance spectroscopy of a nanoporous Li2FeSiO4/C cathode during the initial charge/discharge cycle of a Li-ion battery

Understanding of the structural evolution of the cathode during the charge/discharge processes is crucial to describe the Li insertion/de-insertion mechanisms in a Li-ion battery. An in situ XRD cell has been specially fabricated to study a nanostructured electrode using a standard laboratory diffractometer. This cell was used to investigate phase transformations of a nanoporous Li2FeSiO4/C cathode in the initial charge/discharge cycle by in situ XRD as well as analyzing the full Li-ion battery by electrochemical impedance spectroscopy (EIS). The battery was operated in chronocoulometric mode for the in situ XRD and galvanostatic intermittent titration technique (GITT) mode for the EIS. Coexistence of two different polymorphs, P2(1)/n and Pmn2(1) of Li2FeSiO4, was observed in the in situ XRD patterns. The amount of P2(1)/n phase, which was the only phase present before cycling, decreased while the amount of Pmn2(1) phase increased during the first cycle. In the fully discharged state the Pmn2(1) phase appeared as the main phase. An inductive loop was observed in the impedance spectra which is believed to arise from the formation of a concentration cell (Li vertical bar P2(1)/n parallel to Pmn2(1)vertical bar Li) from which current flows in opposition to the Li being intercalated/de-intercalated into and out of the Li2-xFeSiO4 electrode. (C) 2013 Elsevier B.V. All rights reserved.