Elucidating intercalation of transition metal guests to tailor properties of layered MXenes

Transition metal (TM) cations exhibit distinct electronic properties, compared to alkali and alkaline earth metal cations. In this work, we aim to explore the electronic coupling between different redox-active TMs guest and conductive MXene host. Specifically, within the confined environment of MXene layers, different inserted TM cations demonstrate varying electronic structures and exert distinct influences on the MXene. Unlike Cu which undergoes unusual reduction upon insertion within MXenes, our Bader charge analysis and X-ray absorption spectroscopy (XAS) results indicate that the oxidation states of inserted Ni, Co, Mn, and Zn ions remain unchanged. Despite the higher degree of Ti oxidation led to higher resistivity of intercalated MXenes, the density of states analysis of Ni and Co highlighted the important contributions of these TM cation guests on the overall electrical conductivity of intercalated-MXenes. Moreover, the TM-intercalated MXenes displayed varying redox contributions to the charge storage, where the redox properties of transition metal can be suppressed. This study highlights the unique influence of each transition metal cation under MXene confinement and lays the groundwork for utilizing transition metal guests in MXenes for various electrochemical applications.