Correlative electrochemical acoustic time-of-flight spectroscopy and X-ray imaging to monitor the performance of single-crystal and polycrystalline NMC811/Gr lithium-ion batteries
LiNixMnyCozO2 (NMC) electrodes typically consist of anisotropic single-crystal primary particles aggregated to form polycrystalline secondary particles. Electrodes composed of polycrystalline NMC particles have a comparatively high gravimetric capacity and good rate capabilities but do not perform as well as single crystal equivalents in terms of volumetric energy density and cycling stability. This has prompted research into well-dispersed single-crystalline NMC products as an alternative solution for high-energy-density batteries. Here, for the first time known to the authors, electrochemical acoustic time-of-flight (EA-ToF) spectroscopy has been shown to be effective in distinguishing between Li-ion batteries composed of either single-crystal NMC811 (SC-NMC811) or polycrystalline NMC811 (PC-NMC811) electrodes. Cells composed of PC-NMC811 electrodes had a higher degree of gas evolution compared to cells containing SC-NMC811 electrodes. Cells composed of PC-NMC811 electrodes also underwent larger changes in the acoustic signal's time-of-flight (ToF) during constant current cycling at a range of C-rates indicating expansion, fracture or dislocation of the reflective interfaces inside the cell. In addition, X-ray computed tomography (X-ray CT) has been used to confirm significant morphological differences between SC-NMC811 electrodes and PC-NMC811 electrodes including the electrode's particle size distribution (PSD) that is suggested to have an effect on acoustic signal interaction with these electrode interfaces
