Hierarchical Three-layered TiO2@carbon@MoS2 Tubular Nanostructures as Anode Materials for Lithium Ion Batteries

Lithium-ion batteries (LIBs) have received considerable attention as the power source for portable electronic devices. The anode materials used in LIBs suffer from limitations such as poor intrinsic electronic conductivity, sluggish Li⁺ ion transport kinetics and the inevitable volume change that occurs during the lithium insertion/de-insertion process. To overcome these limitations, researchers at School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore have demonstrated a multi-step synthesis route to prepare hierarchical tubular nanostructures by sequentially coating nitrogen-doped carbon (NC) layer and ultrathin MoS₂ nanosheets on TiO₂ nanotubes (designated as TiO₂@NC@MoS₂).

The multistep process involved in the synthesis of TiO₂@NC@MoS₂ tubular nanostructures is schematically represented in Fig. 1. MnO₂ nanowires (average diameter ≈40 nm) with a high-aspect ratio synthesized by hydrothermal method served as the starting template. A TiO₂ layer was deposited on the MnO₂ nanowires to develop core shell MnO₂@TiO₂ nanowires (step I). A layer of polydopamine (PDA) (thickness: 10 nm) was deposited over the MnO₂@TiO₂ nanowires to produce coaxial MnO₂@TiO₂@PDA nanowires (step II). Subsequently, the MnO₂@TiO₂@PDA nanowires were carbonized at 500 °C for 3 h under N₂ atmosphere followed by acid etching to remove the MnO₂ template (step III). In the meantime, the outer PDA layer is converted into NC shell for the core–shell TiO₂@NC nanotubes (step III). Finally, a layer of ultrathin MoS₂ nanosheets was grown on the surface of TiO₂@NC nanotubes by a hydrothermal reaction, which upon subsequent annealing (H₂/Ar atmosphere at 700 °C for 2 h) yields three-layered hierarchical TiO₂@NC@MoS₂ tubular nanostructures (step IV).

Fig. 1 Schematic of the multi-step synthesis process of TiO₂@NC@ MoS₂ tubular nanostructures: (I) TiO₂ coating; (II) PDA coating; (III) carbonizing and acid etching; and (IV) deposition of MoS₂ nanosheets and annealing.

The morphological features of TiO₂@NC@MoS₂ tubular nanotubes indicate that the hierarchical MoS₂ shell is composed of randomly assembled ultrathin nanosheets (Figs. 2(a) and 2(b)) while the TEM image (Fig. 2(c)) reveals its hollow structure.

Fig. 2 (a, b) FE-SEM; and (c) TEM images of TiO₂@NC@MoS₂ nanotubes

Galvanostatic charge/discharge voltage profiles indicate that the TiO₂@NC@MoS₂ electrode delivers high initial discharge and charge capacities of 1410 and 838 mAh/g, respectively, with a Coulombic efficiency (CE) of 59.4%. Pre-lithiation of TiO₂@NC@MoS₂ electrode is a viable option to bring the initial CE to ~100%. In spite of the low CE, the capacity quickly stabilizes after the 1^st cycle. The coincidence of the discharge–charge curves points out that the electrochemical reactions are highly stable and reversible after the first cycle (Fig. 3(a)). The average specific discharge capacity is decreased from  ≈925 to 612 mAh/g with an increase in current density from 0.1 to 2.0 A/g. However, the capacity of the electrode reverts back to 955 mAh/g when the current density is decreased from 2.0 to 0.1 A/g, thus confirming its good reversibility (Fig. 3(b)). The cycling performance of the TiO₂@NC@MoS₂ electrode indicate that it can retain a high reversible capacity of 590 mAh/g after 200 cycles.

The improved performance of the TiO₂@NC@MoS₂ nanotube electrode is due to synergetic effect of the three functional layers. In the sandwich-like structural arrangement, the inner layer of TiO₂ nanotubes serves as a skeleton of the hybrids, buffers the large volume variation of the electrode for stable cycling performance and shortens the diffusion distance of Li⁺ ions to achieve high rate capacities. The highly conductive N-doped C layer in the middle facilitates electron transfer within the hybrid, protects the overall 1D hollow structure, and prevents the MoS₂ nanosheets from restacking. The outer layer of ultrathin MoS₂ nanosheets with high surface area provides sufficient electrode/electrolyte contact area and reduces the diffusion length for the transfer of electrons and Li⁺ ions to realize a high specific capacity.

Fig. 3 Electrochemical performance of TiO₂@NC@MoS₂ tubular nanotubes for lithium storage: (a) Discharge/charge voltage profiles for the first 5 cycles at 0.2 A/g; and (b) Rate performance at various current densities.

The TiO₂@NC@MoS₂ tubular nanostructures exhibit enhanced lithium storage in terms of high capacity, long cycle life, and good rate performance and hence it can be considered as an effective electrode material for LIBs.

T.S.N. Sankara Narayanan

For more information, the reader may kindly refer: Sibo Wang et al., Rational Design of Three-Layered TiO₂@Carbon@MoS₂ Hierarchical Nanotubes for Enhanced Lithium Storage, Adv. Mater. 2017, 1702724, DOI: 10.1002/adma.201702724