Synthetic, Structural, and Electrochemical Study of Monoclinic Na₄Ti₅O₁₂ as a Sodium-Ion Battery Anode Material

The monoclinic phase of Na₄Ti₅O₁₂ (M-Na₄Ti₅O₁₂) has been investigated as a potential sodium-ion battery anode material. In contrast to the previously investigated trigonal phase (T-Na₄Ti₅O₁₂), M-Na₄Ti₅O₁₂ 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-Na₄Ti₅O₁₂. Neutron powder diffraction reveals the preferred sites and occupancies of the excess sodium. In situ synchrotron X-ray diffraction under electrochemical cycling shows that the crystal lattice undergoes strongly anisotropic volume changes during cycling.