Simultaneous Monitoring of Structural Changes and Phase Distribution of LiFePO4 Along the Cathode Thickness of Li Metal Polymer Battery

Abstract

Probing the structural changes that electrode materials undergo during electrochemical cycling while monitoring their spatial distribution within the volume gives valuable insights on dynamic processes, i.e. side reactions and evolution of phase migration barriers, often associated to capacity and power limitation. In this work, we present an electrochemical cell to perform spatial and time resolved operando synchrotron X-ray diffraction on Lithium (Li) metal polymer batteries operating at 80 °C. A 3.2 mm diameter battery made of a Li metal anode, a LiFePO4 based cathode, and a solid polymer electrolyte acting as separator, is placed inside a glass-based casing and cycled at a beamline. The cylindrical cell geometry with its small size enables to follow the phase transformations occurring at different states of charge and at different cathode heights. It is possible to create spatially resolved phase distribution plots and to differentiate active material structural changes occurring close to the interface with the electrolyte from those at the current collector vicinity. The results provide a direct observation of the Li diffusion in the LiFePO4 and FePO4 phase distribution. In addition, synchrotron X-ray diffraction computed tomography (XRD-CT) measurements were performed to obtain phase distribution maps at different heights of the battery assembly.