Sn-doped MAX phases as anodic materials for lkaline ion batteries

In the scarce landscape of anodic materials, one of the most interesting is the family of MXenes: 2D materials with Mn+1XnTx stoichiometry, where M is a transition metal, X is carbon or nitrogen and T is the functionalization of the layers. However, such materials are obtained by etching the corresponding MAX phase precursor with HF; to prevent this hazardous operation, we studied thermally treated Sn-doped MAX phases directly as anodes both in lithium and sodium ion batteries (LIBs, SIBs). The Ti3Al(1-x)SnxC2 MAX phase samples with x = 0, 0.4, and 0.7 were synthesized via Spark Plasma Sintering (SPS). They were subjected to a thermal treatment at 600°C in air and studied as anodic materials in SIBs and LIBs by galvanostatic cycling with potential limitation (GCPL). The MAX phases have been characterized by X-ray and neutron diffraction (XRD, ND), thermal gravimetric analysis (TGA), CHNS, electron microscopies (SEM, TEM), and Raman spectroscopy. The electrochemical performances of the oxidized samples are noticeably better than the non-oxidized ones: for the NIBs, at 1 C the specific capacity is around 70-78 mAh/g, while for the non-oxidized samples it is around 10-16 mAh/g. The LIBs coins show a remarkably increasing capacity for a higher Sn content: it is 200 mAh/g for Sn0.4_Ox and 250 mAh/g for Sn0.7_Ox at 1 C. The structural analysis in agreement with the Raman analysis and the TEM electron diffraction evidenced the appearance of rutile and Ti-Sn mixed oxides in the thermally treated samples, and the TEM imaging showed the presence of such oxides as nanometric crystallites (diameter of ~20 nm) on the surface of the MAX phase. According to our TGA analysis and the literature, the presence of Sn lowers the oxidation resistance of the MAX phase, resulting in a lower oxidation temperature and a higher percentage of oxides, which is confirmed also by CHNS. Since the MAX phase is inert, the potential profiles from the GCPL suggest that for the LIBs the measured capacity is due to mixed mechanisms of intercalation, conversion, and alloying in the nanostructured oxide composite.