Most cathode materials for lithium-ion batteries exhibit a low electronic conductivity. Hence, a significant amount of conductive graphitic additives are introduced during electrode production. The mechanical stability and electronic connection of the electrode is enhanced by a mixed phase formed by the carbon and binder materials. However, this mixed phase, the carbon binder domain (CBD), hinders the transport of lithium ions through the electrolyte pore network. Thus, reducing the performance at higher currents. In this work we combine microstructure resolved simulations with impedance measurements on symmetrical cells to identify the influence of the CBD distribution. Microstructures of NMC622 electrodes are obtained through synchrotron X-ray tomography. Resolving the CBD using tomography techniques is challenging. Therefore, three different CBD distributions are incorporated via a structure generator. We present results of microstructure resolved impedance spectroscopy and lithiation simulations, which reproduce the experimental results of impedance spectroscopy and galvanostatic lithiation measurements, thus, providing a link between the spatial CBD distribution, electrode impedance, and half-cell performance. The results demonstrate the significance of the CBD distribution and enable predictive simulations for battery design. The accumulation of CBD at contact points between particles is identified as the most likely configuration in the electrodes under consideration.
Due to their extreme volume expansion, Si/C-composites suffer from fracture or delamination and consequent capacity fading during the Li-ion cell operation. One approach to reduce the electrical contact loss and improve the performance is the application of mechanical pressure on the cell. Therefore, a comprehensive aging study of Si/C|NMC811 pouch cells is conducted with cells in different compression configurations as uncompressed and under flexible and fixed compression at pressure levels in the range of 0.08 MPa, 0.42 MPa, and 0.84 MPa. In-situ swelling measurements by dilation as well as in-operando mapping of the pressure distribution on the cell surfaces reveal the positive influence of the low pressure fixed and the middle pressure flexible compression on the cycle life. For the heavily fixed compressed cells, an inhomogeneous pressure distribution and occurring pressure hot -spots close to the cell current collectors of up to 5.2 MPa are found. An extensive post-mortem analysis including SEM cross-sectioning and EIS measurements of the aged anodes and the separator confirms cell failure by different aging mechanisms depending on the type of compression. Aging experiments of Si/C|NMC811 cylindrical 18650-cells show local differences along the jelly roll which are explained by the help of the pouch cell results.
The addition of Si compounds to graphite anodes has become an attractive way of increasing the practical specific energies in Li‐ion cells. Previous studies involving Si/C anodes lacked direct insight into the processes occurring in full cells during low‐temperature operation. In this study, a powerful combination of operando neutron diffraction, electrochemical tests, and post‐mortem analysis is used for the investigation of Li‐ion cells. 18650‐type cylindrical cells in two different aging states are investigated by operando neutron diffraction. The experiments reveal deep insights and important trends in low‐temperature charging mechanisms involving intercalation, alloying, Li metal deposition, and relaxation processes as a function of charging C‐rates and temperatures. Additionally, the main aging mechanism caused by long‐term cycling and interesting synergistic effects of Si and graphite are elucidated.
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