(Invited) MAX Phase Oxidation As a Novel Approach to Highly Reversible Oxide Nanocomposite Electrodes for Lithium-Ion Batteries

Among the materials utilized for the negative electrodes in Li-ion batteries, there is notable interest in oxides that can undergo reactions with Li+ through processes like intercalation, conversion, or alloying. These oxides exhibit high specific capacities but face challenges related to inadequate mechanical stability. A novel approach to constructing nanocomposites involves the partial oxidation of the tin-containing MAX phase within the Ti 3 Al (1-x) Sn x O 2 composition. By employing this strategy, and optimizing the Sn amount and the oxidation process, we have developed composite electrodes comprising Sn/TiO x and MAX phase, demonstrating exceptional durability with over 600 cycles in half cells. These electrodes exhibit charge efficiencies exceeding 99.5% and specific capacities comparable to graphite, surpassing those of lithium titanate (Li 4 Ti 5 O 12 ) or MXene-based electrodes. These remarkable electrochemical performances extend to full cell configurations when combined with a low cobalt content layered oxide. The success of these electrodes is elucidated through a thorough chemical, morphological, and structural investigation, revealing the intimate contact between the MAX phase and oxide particles. Throughout the oxidation process, electroactive nanoparticles of TiO2 and Ti (1-y) Sn y O 2 emerge on the surface of the unreacted MAX phase, which serves both as a conductive agent and a buffer to maintain the mechanical integrity of the oxide during lithiation and de-lithiation cycles.