Synthesis of 2D Li4Ti5O12 Nanosheets via the “Insertion–Exfoliation–Lithiation” Process

Plenty of research efforts have been devoted to developing the anisotropic 2D architectures with intriguing electrochemical and optoelectronic properties. However, the scalable production of nonlayered binary metal oxides with a 2D structure still remains a great challenge. Here, we develop an "insertion–exfoliation–lithiation" process to delaminate the spinel Li4Ti5O12 (LTO) into the thickness of ∼4 nm. After the intercalation of methylamine (MA) into the H2Ti3O7 interlayers, the MA intercalated titanic acid (MA/Ti3O7) exhibits the volume expansion along the stacking direction with the interlayer spacing increasing from an original 7.9 to 10.1 Å. Driven by the acid–base equilibrium and osmotic pressure balance, this crucial step significantly enhances the exfoliation yield of MA/Ti3O7 nanosheets upon the mechanical peeling process. First-principles calculation validates that 1.1 unit of MA has been inserted per mole of H2Ti3O7 described as MA1.1H0.9Ti3O7. In addition, the transmission-mode in situ X-ray diffraction records the real-time phase transition of the as-developed synthetic process, enabling the precise control over the reaction temperature, phase purity, crystallinity of each intermediate, and prevention of the restacking of the LTO nanosheets into the cubic-spinel bulk material. The as-fabricated LTO nanosheets with an appealing ultrathin structure exhibit a reversible specific capacity of 210 mA h g–1 at 0.25 C, which far surpasses the theoretical capacity limit of bulk counterparts due to the pseudocapacitive contribution from the reduced dimensionality.