EIS Study of the MWCNT Effect in MWCNT@TiO2 Anodes for Li‑ion Batteries

Nowadays, Li‑ion batteries are considered as the most prominent alternative to be used in diverse applications such portable electronic devices [1]. Nevertheless, most of the likely materials to be employed as electrodes in these cells are limited by low Li⁺ ion diffusion and scarce electron transport. Composite materials based on carbon nanotubes and TiO₂ have shown good results for this particular application, exhibiting improved rate capability during cycling, a major drawback of pristine TiO₂ [2]. Most of the electrochemical characterization of these composites have been carried out by direct current techniques, obtaining information only of the overall performance of the material, knowing few about the different steps involved during the charging‑discharging process of the anode [2‑6].

 Herein, MWCNT@TiO₂ composites were synthesized by controlled hydrolysis of titanium isopropoxide over the MWCNT [3], and the interaction between MWCNT and TiO₂ in MWCNT@TiO₂ anodes during the charging‑discharging process was characterized with EIS in order to understand the specific role played by each material.

 The TiO₂ shell formed over the MWCNT was around 100 nm thick, containing 10 nm anatase crystallites. MWCNT@TiO₂ shows higher capacities at every charging discharging rate with a better rate capability than pristine TiO₂. The experimentally obtained EIS spectra were fitted to the equivalent circuit previously proposed for the lithiation‑reaction model [7]. From the fitting it was possible to obtain information of the different processes taking place during the charge and discharge of the battery at steady‑state. Two of the most representative parameters are the charge transfer resistance (R_tc) detected at high frequency, and the chemical capacity (C_µ) detected at low frequency range. The presence of MWCNT in the composite considerably diminished the R_tc of the TiO₂, maintaining it at an almost constant value during the charge‑discharge process. Additionally, the C_µ that is closely related to the Li⁺ ion storage capacity of the material, show similar values for both pristine TiO₂ and MWCNT@TiO₂, indicating that the Li⁺‑ion insertion in the composite material is only taking place in the TiO₂. However, it is worth mentioning that a lower potential is needed to carry the lithium ion insertion into the TiO₂ lattice when it is grown over MWCNT.

 The improvement in the TiO₂ rate capability offered by the MWCNT@TiO₂ composite seems to be related to the increase in the electronic transport, diminishing the R_tc and depolarizing the Li⁺ ion insertion in the TiO₂. The results obtained can be extrapolated to all the family of core‑shell materials based on carbon nanotubes that recently have caught the eye of different research groups working in the developing of efficient materials for Li‑ion batteries [8‑10].

 References

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