The Dynamics of the Structure of Composite Electrodes during their Operation in Lithium‐Ion Batteries

Mechanical and electrochemical experiments are used to infer changes in composite electrodes during cell operation. Macroscopic stress levels are determined by operando substrate curvature measurements of LiFePO 4 (LFP) electrodes containing different types of binders. A reduction in the stress oscillation within a few cycles indicates that the benefits of calendering can be quickly diminished due to structural changes in the electrode. The use of a relatively stiff PAA binder allows for detailed observations of the phase transition during (de)lithiation of LFP, while softer, more viscous binders lead to significantly reduced and blurred macroscopic stresses. To separate mechanical from electrochemical contributions to the electrode mechanics, electrodes are tested with a stress‐controlled compression setup, which mimics the macroscopic stress states during electrochemical cycling. This experiment reveals differences in the evolution of strain and electrode resistance, which are consequences of different particle rearrangement processes. Their motion is linked to the mechanical properties of the binder, which highlights its decisive role in the resulting mechanical and time‐dependent properties of composite electrodes. The results of this work demonstrate that composite electrodes cannot be considered stationary during operation. Electrodes structurally change and develop towards “steady‐state” configurations depending on the operating conditions and the externally imposed load.