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ALD Al2O3-Coated TiO2 Nanotube Layers as Anodes for Lithium-Ion Batteries

Abstract : The utilization of the anodic TiO 2 nanotube layers, with uniform Al 2 O 3 coatings of different thicknesses (prepared by atomic layer deposition, ALD), as the new electrode material for lithium-ion batteries (LIBs), is reported herein. Electrodes with very thin Al 2 O 3 coatings (∼1 nm) show a superior electrochemical performance for use in LIBs compared to that of the uncoated TiO 2 nanotube layers. A more than 2 times higher areal capacity is received on these coated TiO 2 nanotube layers (∼75 vs 200 μAh/cm 2) as well as higher rate capability and coulombic efficiency of the charging and discharging reactions. Reasons for this can be attributed to an increased mechanical stability of the TiO 2 nanotube layers upon Al 2 O 3 coating, as well as to an enhanced diffusion of the Li + ions within the coated nanotube layers. In contrast, thicker ALD Al 2 O 3 coatings result in a blocking of the electrode surface and therefore an areal capacity decrease. ■ INTRODUCTION During the last decade, TiO 2 nanomaterials have widely been studied as an alternative electrode material for lithium-ion batteries (LIBs). 1−15 TiO 2 has a higher lithiation potential (∼1.6 V vs Li/Li +) compared to that of the negative electrodes, such as graphite (∼0.1 V vs Li/Li +), and therefore enhances safety of the cells and provides a good capacity retention on cycling and a low self-discharge. 1−4 Furthermore, TiO 2 shows low volume changes of less than 4% upon reversible insertion and extraction of Li + in the lattice. However, TiO 2 also has a poor electrical conductivity and shows limited Li + uptake and slow Li + insertion kinetics. 1−4 To overcome these drawbacks, nanostructured TiO 2 , such as mesoporous microspheres, 5 nanowires, 6,7 nanoparticles, 8 nanoflakes, 9 or TiO 2 nano-tubes, 1−4,10 produced via different methods have been considered as anodes due to the larger specific surface area. Although most of these structures are randomly oriented and have to be deposited on the back contact of the electrode, self-organized TiO 2 nanotube layers produced by electrochemical anodization of the Ti substrates consist of straight and vertically aligned nanotubes in direct electrical contact with the underlying Ti substrate. 16 Another advantage of the vertically aligned nanotube layers is a direct diffusion path for the Li + ions, resulting in a superior electrochemical performance. 11 To further increase the capacity and conductivity of the TiO 2 nanotubes for their use in LIBs, they have been decorated or coated with other metals and metal oxides, with higher conductivities and capacities, for example, Ag, 17 Cu 6 Sn 5 , 18 Fe 2 O 3 , 19 SnO, 20 SnO 2 , 21,22 or ZnO. 23 Another advantage of these composite structures is that due to their hollow tubular architecture they can bear volume variations upon battery cycling without mechanical failure. Among the various methods that can be used for the coating and decoration of the TiO 2 nanotube layers, atomic layer deposition (ALD) has been the only method that creates uniform coatings of the nanotube walls from inside as well as outside, with a precise control of the coating thickness according to the deposition cycles. 24−26 However, although several reports can be found on utilization of the ALD coatings of different materials to modify the electrodes for lithium-ion battery application, 27−33 only one publication reports on the use of ALD to coat an anode prepared from an anodic TiO 2 nanotube layer with ZnO. 23 Usually, passivation layers on the electrode surfaces (solid electrolyte interphase (SEI)) are formed via decomposition of
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Hanna Sopha, Girish D. Salian, Raul Zazpe, Jan Prikryl, Ludek Hromadko, et al.. ALD Al2O3-Coated TiO2 Nanotube Layers as Anodes for Lithium-Ion Batteries. ACS Omega, ACS Publications, 2017, 2 (6), pp.2749 - 2756. ⟨10.1021/acsomega.7b00463⟩. ⟨hal-01813840⟩

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