Irreversible structural change of a spinel Li4Ti5O12 particle via Na insertion-extraction cycles of a sodium-ion battery

•Spinel Li4Ti5O12 as a Na-ion battery anode material was studied by XRD and TEM.•Irreversible structural change of Li4Ti5O12 was induced during the cycles.•Extra phase of a larger lattice volume than pristine Li4Ti5O12 was confirmed by XRD.•Extra phase was identified by TEM as a Na-rich domain in the Li4Ti5O12 particle.•Na-rich phase should also participate in the Na insertion-extraction reaction.

We investigate the structural changes of spinel lithium titanate (LTO; Li4Ti5O12) via the electrochemical Na insertion-extraction cycles, using X-ray diffraction (XRD) and analytical transmission electron microscopy (A-TEM). The XRD reveals that an extra phase with an about 4‑5% larger unit cell volume is generated in pristine LTO during the first cycle. The A-TEM reveals that the extra phase co-exists with Li4Ti5O12 in a single particle and is identified as a Na-substituted LTO phase (Na-LTO; Li4-xNaxTi5O12, 0 < x < 3). The reversible electrochemical capacity of the electrode (155 mA h g−1) is more than 90% of the theoretical one, suggesting that both LTO and Na-LTO are involved in the charge-discharge reactions.

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