MWCNT TiO2 Composite Films for Li Ion Batteries Anodes with Improved Rate Capability

The development of high rate capability electrodes for Li‑ion batteries is a priority topic in order to diversify the applicability of this dispositive. High capacity materials usually fail when the charge‑discharge cycle is carried out at high rates, due to scarce electronic conductivity, high volume expansion, low Li‑ion diffusion coefficient in the solid, among others [1‑3]. Composites with superior properties than its raw materials can be achieved by bringing together the best properties of each component in order to enhance its functionality. Particularly, matrixes with high electronic conductivity to allow the efficient transport toward the current collectors, covered or decorated with materials that can efficiently support the Li‑ion insertion‑extraction process, have shown to be an adequate strategy to avoid or diminish the capacity shrink commonly observed when the material is charged‑discharged at high rates [4‑6].

 Herein, MWCNT‑TiO₂ composites films were obtained over stainless steel discs by a three steps process. Firstly, the home‑made MWCNT were deposited onto the substrate by spray, then different layers (1, 3, 5 and 10) of TiO₂ were formed by dip‑coating, and finally the films were heat treated at 400°C in order to obtain the anatase polymorph. This approach avoids the use of polymeric binders that may decrease the electronic conductivity of the film, and allows a direct contact of the composite with the substrate. The morphology and structural properties of the obtained films were characterized by XRD and SEM analysis, and their electrochemical performance tested in two‑electrodes cell. The electrolyte used was 1.0 M LiPF₆ in a 50:50 (w/w) mixture of ethylene carbonate and diethyl carbonate, and a glass fiber from Whatman was used as a separator.

 The CV characterization showed the typical current peaks for the Li‑ion insertion (cathodic) and extraction (anodic) in the TiO₂. For the films with low TiO₂ content (1 and 3 layers) the current peak do not decrease with the number of scans, as it happen for the film with high TiO₂ content (10 layers). Additionally, the potential difference between cathodic and anodic peaks shows an increase with the number of TiO₂ layers in the film. On the other hand, despite the materials showed similar capacities at low rates, the capacity of the film decreases with the charging‑discharging rate, as the TiO₂ covering the MWCNT becomes thicker (e.g. higher TiO₂ layers). The observed behavior was related to the low electronic conductivity of the TiO₂, requiring higher times for electron transport and higher path for Li‑ion insertion/extraction compared to those films with a thinner TiO₂ layer.

 References

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