Materials for alkaline ion batteries based on MAX phase and related compounds towards a pre-industrial scale up

The global need to decarbonize energy systems and enable large-scale integration of renewable sources has placed rechargeable batteries at the core of modern technological development. While lithium-ion batteries (LIBs) have achieved unparalleled success in portable electronics and electric mobility, their further expansion is constrained by the scarcity and geopolitical concentration of critical raw materials, as well as limitations in energy density for today's demand. In this context, the exploration of novel electrode materials that can ensure high capacity, structural robustness, and environmental compatibility, with the possibility to employ other energy storage systems with respect to LIBs, represents a central scientific and technological challenge. Here is presented a comprehensive investigation of Sn- and Si-based MAX phases and their oxidized derivatives as innovative negative electrodes for both lithium- and sodium-ion batteries (LIBs and SIBs). MAX phases (layered ternary carbides and nitrides with the general formula Mn+1AXn) combine metallic and ceramic attributes, providing a unique structural platform to engineer composite electrodes with high electrical conductivity and mechanical resilience. Through controlled oxidation of Ti3SnxAl(1-x)C2 (312-type) and Ti2SnxAl(1-x)C (211-type) systems, it was possible to generate nanocomposites composed of conductive MAX cores and finely dispersed Ti/SnO2 active domains, whose intimate interfacial contact promotes efficient electron transport and mitigates volumetric strain during conversion-alloying reactions. An extensive suite of advanced characterization techniques, including operando synchrotron X-ray diffraction (SXRD), X-ray absorption spectroscopy (XAS), ¹¹⁹Sn Mössbauer spectroscopy, Raman spectroscopy, and high-resolution electron microscopy (SEM, TEM, STEM-EDX), was employed to elucidate the structural evolution and oxidation mechanisms of these systems. The results reveal a controlled transformation pathway leading to hierarchical architectures with optimized phase composition and nanoscale morphology, directly correlated with their electrochemical behavior. Electrochemical testing demonstrated that oxidized MAX phases deliver high reversible capacities, excellent rate capability, and superior cycling stability in both Li⁺ and Na⁺ systems. In particular, the hybrid structure enables the coexistence of reversible alloying within a mechanically buffered matrix, leading to mechanical stability and, as a consequence, high coulombic efficiency over extended cycling. Operando analyses confirmed the mechanism of conversion and (de)alloying that involved the SnO2 and the structural preservation of the MAX-derived conductive framework. The research was further extended to Si- and Si/Sn-based MAX phases, whose oxidation products demonstrated enhanced Li⁺ storage capability, confirming the potential of this strategy to obtain electrochemically active composite materials. Finally, Sn@MXene composite scaffolds were developed and evaluated as hosts for Li-metal anodes, demonstrating that the synergistic effect between the structure of MXene and the nature of tin effectively stabilizes lithium plating/stripping behavior. Collectively, this work establishes MAX phases as a versatile materials platform to obtain new promising materials. The insights gained into their oxidation chemistry, structure-property correlations, and electrochemical performances provide a solid foundation for the rational development of sustainable electrode materials for next-generation lithium and sodium energy storage technologies.