Layered Na 2 Ti 2 O 4 (OH) 2 and K 2 Ti 2 O 4 (OH) 2 Nanoarrays for Na/Li-Ion Intercalation Systems: Effect of Ion Size

Abstract

Vertically oriented Na2Ti2O4(OH)2 (NTO) and K2Ti2O4(OH)2 (KTO) nanoarrays on Ti foils are synthesized via one-step hydrothermal method. These titanates-based layered materials with different cations are utilized to assemble binder-free Li/Na- ion batteries (LIBs/NIBs). We experimentally find the ion size difference of K, Na and Li in electrode materials or/and electrolyte can dramatically influence the performances of LIBs/NIBs. NTO with larger volume of unit cell shows higher capacity in both NIBs (213 mA h g−1 vs. 165 mA h g−1) and LIBs (509 mA h g−1 vs. 288 mA h g−1). That is, the ion size in host materials can significantly influence the intercalation/extraction of Na and Li ions. On the other hand, Na ions with larger size and atomic mass in the electrolyte show slower charge transfer (81.5-144.9 μs vs. 0.23 μs) and ion diffusion (7.52 × 10−10 cm2 s−1 vs. 1.33 × 10−9 cm2 s−1, Na2Ti2O4(OH)2-based cells) than that of Li ions. This work clearly demonstrates the influence of ion size, which can clarify and provide fundamental information for fabricating high energy density and long-life batteries. In addition, it demonstrates that both Na2Ti2O4(OH)2 and K2Ti2O4(OH)2 can be used as anode materials with around two ions storage, as for LIBs (4.6 for Na2Ti2O4(OH)2 and 2.9 for K2Ti2O4(OH)2), and NIBs (1.9 for Na2Ti2O4(OH)2 and 1.7 for K2Ti2O4(OH)2). Moreover, it is demonstrated that Li/Na ions are stored between the adjacent layered sheets in Na2Ti2O4(OH)2 and K2Ti2O4(OH)2.