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

Abstract

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, P21/n and Pmn21 of Li2FeSiO4, was observed in the in situ XRD patterns. The amount of P21/n phase, which was the only phase present before cycling, decreased while the amount of Pmn21 phase increased during the first cycle. In the fully discharged state the Pmn21 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|P21/n||Pmn21|Li) from which current flows in opposition to the Li being intercalated/de-intercalated into and out of the Li2−xFeSiO4 electrode.