Interlayer modulation of the lepidocrocite-type titanate via intercalation of the tetraalkylammonium ions for high-performance sodium-ion batteries

Sodium-ion batteries (SIBs) are an alternative to lithium-ion batteries (LIBs) due to the low cost. However, the large ionic radius of Na-ion hinders their efficient diffusion within the hosts of SIBs. Interlayer expansion of intercalation hosts is a option to increase the mobility of sodium ions. In this paper, tetraalkylammonium ions of different chain lengths were inserted into the lepidocrocite-type titanate to obtain interlayer modulation for investigation of their potential as anodes in SIBs. Surprisingly, the biggest interlayer spacing produced by the longer chain length of tetrabutylammonium ions does not give the best performance. The tetraethylammonium intercalated layered titanate surpasses others with a specific capacity of 175 mAh g⁻¹ at 0.1 A g⁻¹. The phenomena could be explained by variation of the charge transfer resistance and and diffusion coefficient induced by changing guest ion species. This work provides a useful guide for developing high-efficiency layered electrode materials for SIBs.