Synthetic, Structural, and Electrochemical Study of Monoclinic Na<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> as a Sodium-Ion Battery Anode Material

The monoclinic phase of Na<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> (M-Na<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub>) has been investigated as a potential sodium-ion battery anode material. In contrast to the previously investigated trigonal phase (T-Na<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub>), M-Na<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub> has continuous two-dimensional (2D) channels with partially occupied Na sites, providing broader pathways and more space for the intercalation of excess sodium. Electrochemical measurements show that it exhibits a comparable or higher reversible capacity than T-Na<sub>4</sub>Ti<sub>5</sub>O<sub>12</sub>. Neutron powder diffraction reveals the preferred sites and occupancies of the excess sodium. <i>In situ</i> synchrotron X-ray diffraction under electrochemical cycling shows that the crystal lattice undergoes strongly anisotropic volume changes during cycling.