Electrochemical Synthesis of Potassium Titanate Nanowires in Molten Salts with Good Li⁺-Intercalation Performance

Abstract

The novel material K 1.35 Ti 8 O 16 @C was successfully fabricated via an effective, controllable, and cost-effective electrochemical synthetic method and further evaluated as an anode material for lithium-ion batteries (LIBs). This material delivered high reversible capacities of ∼320 mAh g -1 at 50 mA g -1 and excellent long-term cycle stability (135 mAh g -1 after 2000 cycles at 1 A g -1 ) owing to the high electronic conductivity and fast reversible electrochemical Li-intercalation/extraction behavior of the nanowire structure of K 1.35 Lithium ion batteries (LIBs) are promising electrochemical power sources owing to their high output voltage, energy density, and long calendar life.1 In order to enhance their capacity and power density, several new candidates such as transition metal oxides, 2 alloy-based materials, 3 and sulfides 4 have been explored as next-generation anode materials for commercial LIBs. Among the currently used anodes for LIBs, TiO 2 is the most prevalent owing to its low cost, environmentalfriendliness, structural stability, and better insertion kinetics.5 However, the large polarization at high charge/discharge rates because of the sluggish Li + diffusion and poor electron transport impedes the practical application of TiO 2 in LIBs. 6,7 Hollandites are very well known mainly because of the performance of α-MnO.8 However, the ability to accept lithium ions via an intercalation reaction of this compound is too high to be considered as a suitable candidate for anode material. 9 As one of polymorphs of titanium dioxide, hollandite-type TiO 2 is composed of edge-shared TiO 6 -octahedral units interlinked as a one-dimensional tunnel structure. The hollandite-type TiO 2 and the similar hollandite-type K cations) occupy the tunnel and preserve the charge balance. 12 Owing to the unique microporous structure, the cations can be easily inserted into and extracted from the tunnel while the overall original structure remains intact.The electrochemical synthetic method is a simple, fast, and controllable preparation route which can be used to obtain different metal oxides with different valence states. 13,14 In this work, the electrochemical synthesis of K 1.35 Ti 8 O 16 via electrolysis of solid TiO 2 in molten KCl salt has been described. To the best of our knowledge, this is the first report in literature to develop this synthetic technique. The structure and morphology of the as-prepared K 1.35 Ti 8 O 16 was studied. Furthermore, it was applied as an anode material of LIBs and showed excellent performance in Li + storage. ExperimentalSynthesis of the TiO 2 @C precursor.-The TiO 2 @C composite was prepared by a facile hydrothermal method, as schematically illustrated in Figures 2a-2c. TiCl 4 (1 mL) was dissolved in 80 mL of ethylene glycol by continuous stirring until the solution was clear. Ammonium hydroxide (2 mL, 25%) was then added into the solution and the mixture was stirred for 10 min. This solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and kept in the ov...