The structural behavior of electrochemically delithiated Li_xNi_0.8Co_0.15Al_0.05O₂ (x<1) battery cathodes

A full series of variously delithiated Li_xNi_0.8Co_0.15Al_0.05O₂ (NCA) (x < 1) battery cathodes is extracted from cylinder Li-ion batteries of 18650-type, and structural studies of electrode materials are performed ex situ applying high-resolution neutron powder diffraction. A set of structural and microstructural parameters at different state-of-charges (SOC) is gathered from full-profile Rietveld refinements. Accurate cell metrics of the NCA lattice are obtained and compared for both, ex situ and operando modes of data collection. A detailed analysis of lithium occupancies and interatomic distances in the NCA cathode is carried out, revealing a shrinking material response of the NCA structure during delithiation, while the experimental quantification of Li/Ni cation mixing showed an SOC independent behavior. The observed deintercalation driven anisotropic broadening of the Bragg reflections in real-life NCA cathodes were analyzed and assigned to an increasing internal anisotropic microstrain, which is linked to the non-uniform distribution of Li in the structure of NCA, resulting in an unstable and unpredictable performance of LIBs.