Layered Na₂Ti₂O₄(OH)₂and K₂Ti₂O₄(OH)₂Nanoarrays for Na/Li-Ion Intercalation Systems: Effect of Ion Size

Abstract

Vertically oriented Na 2 Ti 2 O 4 (OH) 2 (NTO) and K 2 Ti 2 O 4 (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 cm 2 s −1 vs. 1.33 × 10 −9 cm 2 s −1 , Na 2 Ti 2 O 4 (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 Na Energy conversion and storage is a key issue in daily life and industry field. The urgent need of renewable and clean energy has attracted great attention in low-cost, safety and rechargeable battery with adequate voltage, capacity and rate capacity.1 Nowadays, Li ion battery (LIB) has conquered the portable electronic market, offering the largest energy density and output voltage of all rechargeable batteries in use.2 However, the amount of storage on earth and higher cost to obtain Li has become a critical barrier to popularization and sustainable application in battery energy storage. 1,3 Due to the wide availability and low-cost of sodium, Na ion battery (NIB) is a promising alternative to LIB in the future. But sodium has a lower reducing voltage (-2.71 V vs. SHE, compared to -3.04 V) and the gravimetric capacity is lower (1165 mA h g −1 compared to 3829 mA h g −1 ) than lithium. 4 The atomic mass of Na is more than three times larger and the ionic radius of Na is 0.3 Å larger than Li. Furthermore, graphite cannot be used as an anodic insertion host for Na ions. 5,6 Hence, Na-based anodes are urgently needed for the development of NIBs.To 20 Amazingly, this anode shows Na ion storage performance is much better than that in LIBs. The authors attributed this to slower ion transfer in LIBs than in NIBs, which differs from the general knowledge on the size/atomic mass effects of Na and Li. Therefore, it is very important to investigate/clarify how the ion size in electrode materials or/and electrolyte affects the performance of NIBs/LIBs. Na 2 Ti 2 O 4 (OH) 2 (NTO) is a layered sodium titanate with the bodycentered orthorhombic crystal structure, which can be synthesized through one-step hydrothermal method.21,22 NTO is made up of TiO 6 z E-mail: hong1979@swu.edu.cn octahedral that share edges to form two dimensional sheets (Fig. S1a). 21,23...

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