Operando phase transition mapping of the negative electrode of a Li-ion 18 650 battery at high C-rates through fast synchrotron XRD-CT measurements†

Abstract

Lithium-ion batteries (LIBs) have become indispensable in everyday devices and are now being widely used in electric vehicles (EVs) due to their high energy density and long cycle life. However, these batteries are not without their limitations and face various degradation mechanisms that can impact their performance and safety. As the demand for more reliable and efficient batteries grows, it becomes crucial to understand these degradation mechanisms and develop strategies for improving the design and operation of LIBs. Therefore, degradation mechanisms have to be investigated to improve and re-design the commercial devices with the goal of enhancing their capacity, improving their safety and performances. Subsequently, the basic of understanding on the mechanisms involved in the degradation throughout the cycling life of LIBs became a crucial focal point for the research. In situ/operando studies are experiments that involve monitoring the behaviour of a system under realistic operating conditions. This analysis can provide valuable insights on the aging and degradation process on the battery materials and devices. In this work, fast in situ/operando X-ray diffraction computed tomography experiments have been conducted on MJ1-18650 commercial cells during non-stop high C-rate cycling. The goal was to map the changes occurring in the negative electrode of an aged cell during cycling by comparing the phase transitions and lithiation distribution of electrodes in an aged cell to those in a pristine cell. Moreover, comparisons between low and high C-rates were analysed to better understand the influence of the chosen current rate on lithiation distribution inside the cell. During the experiment, the pristine cell showed a uniform phase transition across the volume, with a homogeneous lithiation distribution for both electrodes in charged and discharged states. The aged cell showed a high degree of degradation and deformation after 1200 charge/discharge cycles, with a considerable capacity loss of 14.5%. During the experiment, the aged cell showed an inhomogeneous distribution of lithiation states at both charged and discharged states. The phase transition within the electrode was affected by the rate at which current was delivered to the cell, and it was discovered that at high rates, there were many lithiation states coexisting rather than the phase transition being uniform across the volume. High discharge rates have an impact on graphite phase transitions from the cell's initial condition. The cell was considerably more impacted by current rates after ageing, exhibiting a more marked co-existence of lithiation stage across the volume. The deterioration of the cell certainly had an effect on the phase transition: the divergent dynamics between the centre of the cell and the outside body were more pronounced than in the pristine cell, and the aged cell was unable to attain a fully delithiated or lithiated state at any C rate.