To date, the study of conversion type reactions at both the nanometer and electrode scales over long term cycling still remains very challenging. Here, we show that combining Auger imaging with electrodes cross-section preparation by ion-milling is a suitable tool to study the aging mechanism of conversion type electrodes. Importantly, for NbSnSb-based electrodes, cycled at 25 °C and 60 °C without and with additives, this innovative approach reveals a micron scale spreading of the NbSnSb particles. It thus proves the electrochemical conversion reaction even after 400 cycles and highlights the role of the inactive Nb element in the long term reversibility, which was unexpected. Interestingly, a shell to core expansion/break up of NbSnSb particles is observed and the large and highly porous structures formed led to the electrodes porosity filling and electrodes thickness increase. The formation of the SEI is located inside the porous NbSnSb shells. At 25 °C without additive, this phenomenon is so important that the ionic and/or electronic conductivities are completely altered, which explains the rapid capacity drop to zero. Overall, cross-section Auger imaging will benefit not only to the study of conversion/alloying based batteries but also to all-solid-state batteries aging mechanism for which buried interfaces have to be reached.