Thermal Annealing and Graphene Modification of Exfoliated Hydrogen Titanate Nanosheets for Enhanced Lithium-ion Intercalation Properties

Abstract

Hydrogen titanate has been considered as a promising lithium intercalation material due to its unique layered structure. In the present work, we fabricate 2D graphene/hydrogen titanate hybrid nanosheets for application as anode materials in lithium-ion batteries. H2Ti3O7 nanosheets are synthesized by exfoliation of a layered precursor via interacting bulky tetrabutylammonium (TBA+) cations, followed by ion exchange with Na+ ions and washing with water. The as-prepared hydrogen titanate nanosheets are well-dispersed exhibiting ultra-thin thickness with a lateral size up to a few micrometers. The sample is then annealed at 450, 650 and 850 °C, to optimize its Li+-intercalation property. Heating at 450 °C leads to well-crystallized hydrogen titanate with a trace amount of TiO2. Heating at 650 and 850 °C results in mixed sodium titanates, since some sodium ions in the interlayer structure cannot be washed away and become chemically bonded to [TiO6] octahedra at high temperatures. Electrochemical properties of all the four samples are then evaluated by charged/discharged for 100 electrochemical cycles at 0.01–2.5 V vs. Li+/Li at a specific current of 170 mA g−1. The unannealed hydrogen titanate delivers the highest initial discharge capacity of 130.5 mA h g−1, higher than 124.6 mA h g−1 from hydrogen titanate annealed at 450 °C, as well as 101.3 and 63.8 mA h g−1 from hydrogen titanate annealed at 650 and 850 °C, respectively, due to the high surface area from well-dispersed unannealed nanosheets. However, after 100 electrochemical cycles, well-crystallized hydrogen titanate annealed at 450 °C retain the highest charge capacity of 115.2 mA h g−1, corresponding to a capacity retention of 92.5%, while unannealed hydrogen titanate exhibits a final capacity of 72.1 mA h g−1 and a capacity retention of only 55.2%. To further improve energy density of lithium-ion battery, graphene/hydrogen titanate hybrid nanosheets are fabricated by adding graphene nanosheets into hydrogen titanates. The initial charge capacities of unannealed and annealed hydrogen titanate at 450 °C are significantly increased to 170.7 and 233.9 mA h g−1, respectively. A charge capacity of 101.0 mA h g−1 is retained for unannealed hydrogen titanate with graphene-modification after 100 electrochemical cycles since well-dispersed hydrogen titanate nanosheets can be mixed with 2D graphene more uniformly and thus facilitates diffusion of Li+ ions and retard aggregation of active materials.