  The degradation mechanism in a sodium cell of a layered
Na0.48Al0.03Co0.18Ni0.18Mn0.47O2 (NCAM) cathode with P3/P2 structure is
investigated by revealing the changes in microstructure and composition upon
cycling. The work aims to rationalize the gradual performance decay and the
alteration of the electrochemical response in terms of polarization, voltage
signature, and capacity loss. Spatial reconstructions of the electrode by X-ray
computed tomography at the nanoscale supported by quantitative and qualitative
analyses show fractures and deformations in the cycled layered metal-oxide
particles, as well as inorganic side compounds deposited on the material. These
irreversible morphological modifications reflect structural heterogeneities
across the cathode particles due to formation of various domains with different
Na+ intercalation degrees. Besides, X-ray photoelectron spectroscopy data
suggest that the latter inorganic species in the cycled electrode are mainly
composed of NaF, Na2O, and NaCO3 formed by parasitic electrolyte decomposition.
The precipitation of these insulating compounds at the electrode/electrolyte
interphase and the related structural stresses induced in the material lead to
a decrease in cathode particle size and partial loss of electrochemical
activity. The retention of the NCAM phase after cycling suggests that
electrolyte upgrade may improve the performance of the cathode to achieve
practical application for sustainable energy storage.
