Quick charging of Li-ion batteries is often accompanied by rapid expansion of composite battery electrodes, resulting in the appearance of transient stresses inside the electrodes' bulk. Although predicted theoretically, they have never been tracked by direct in situ measurements. Herein, using multiharmonic electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D), acoustic images of strong transient deformations in LiFePO 4 electrodes were obtained in the form of giant resonance frequency and resonance width shifts. The formation of cracks was verified by scanning electron microscopy. The effects of charging rate, stiffness of the polymeric binder, and solution concentration have been identified. The attractive feature of EQCM-D is its high sensitivity for selective probing of average mechanical characteristics of the operated electrodes, especially of the particle−binder interactions, directly linked to the electrode cycling performance. Using EQCM-D, an inexpensive, simple, and fast method of structural health monitoring for battery electrodes can be intelligently designed.

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It is well-accepted that massive cracks in Ni-rich cathode secondary particles are the determining factors for long-term performance degradation; however, the corresponding crack generation and the state of dynamic propagation are still unknown. In this work, we utilize in situ scanning electron microscopy to reveal the dynamical morphological evolution of a single LiNi0.8Mn0.1Co0.1O2 secondary particle embedded in a cathode blend during electrochemical cycling. These observations show that very few cracks appear in the particle when cycled at a normal cutoff voltage of 4.1 V, but when the cutoff voltage is increased to 4.7 V as an extreme working condition, several cracks were clearly initially generated in the core region and propagated radially along the grain boundaries, finally reaching the particle’s surface. Impressively, crack propagation follows a repeat “grow–stagnate–grow” phenomenon during charge–discharge cycling. Our direct in situ investigation provides a full map of crack evolution in the cathode under electrochemical cycling during early stages.

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