Analytical high resolution transmission electron microscopy has been used to clearly evidence the electrochemical reactivity of Cr2O3 thin films versus lithium. We showed that during the reduction process, chromium sesquioxide (+III) transforms first into chromium monoxide (+II) and then into metallic chromium nanoparticles embedded into a Li2O matrix at 0V. On the subsequent recharge up to 3V, we did not convert back to Cr2O3, but only to a chromium monoxide implying a partial re-oxidation process, thus explaining the irreversibility measured during the first cycle. We extended this study to the comprehension of the electrochemical performances of Cr2O3-based electrodes obtained by thermal treatment, under different atmospheres, of chromium rich stainless steel disks. In addition to the characterization of the particles forming the so-obtained electro-active layers, we showed, using nano-probe EDS, that a mixed “Fe–Cr–O” oxide could react versus lithium, through a conversion reaction mechanism, leading to alloyed metallic nanoparticles upon reduction and nanograins of mixed oxide during the following oxidation.