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

• Effect of surface area and crystallinity on lithium storage mechanism was investigated.
• Increased crystallinity increases the diffusivity of lithium ions during lithiation.
• Storage mechanism changes from diffusion to pseudocapacitance with rate increase >4C.
• When diffusion-based storage is predominant increases in polarization resistance occur.
• Increased polarization causes breakdown of electrolyte and electrode failure.

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 m2 g−1), synthesized via a novel continuous aqueous process and annealed at different temperatures (200 °C, 300 °C, and 400 °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 °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 °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 °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.

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