On the complex interplay of crystallinity and surface area effects on Li-ion intercalation and pseudocapacitive storage properties of nanocrystalline anatase

Lithium insertion in anatase is studied by considering simultaneously the effects of crystallinity and surface area on storage mechanism, namely intercalation vs. pseudocapacitive. 6 nm anatase crystallites with high surface area (222 m(2) g(-1)), synthesized via a novel continuous aqueous process and annealed at different temperatures (200 degrees C, 300 degrees C, and 400 degrees C), were electrochemically tested. By annealing, crystallinity was increased, while surface area decreased allowing for the investigation of the contributions of each toward lithiation and delithiation behavior. The as-synthesized and best-annealed (at 300 degrees C) samples were compared at various rates. At low rate (1C and 2C) the annealed anatase had higher reversible capacity than the as-synthesized, due to increased diffusion-based storage. At high rate however, as-synthesized anatase had much higher capacity due to increased surface area and ability to store Li-ions pseudocapacitively. Increased crystallinity leads to increased diffusivity as determined by electrochemical impedance spectroscopy, explaining why the 300 degrees C anneal had higher capacity at low rate. Long-term cycling at high rate, however; showed that reliance on diffusion-based storage (intercalation) in the case of the 300 degrees C anneal causes increased polarization that leads to electrode performance decline. These results point to the importance of simultaneous nanoanatase property (crystallinity and surface area) optimization for achieving stable performance. (C) 2014 Elsevier B.V. All rights reserved.