Abstract
Chemisorbed water and solvent molecules and their reactivity with components from the electrolyte in high-surface nano-structured electrodes remains a contributing factor toward capacity diminishment on cycling in lithium ion batteries due to the limit in maximum annealing temperature. Here, we report a marked improvement in the capacity retention of amorphous TiO2 by the choice of preparation solvent, control of annealing temperature, and the presence of surface functional groups. Careful heating of the amorphous TiO2 sample prepared in acetone under vacuum lead to complete removal of all molecular solvent and an improved capacity retention of 220 mAh/g over 50 cycles at a C/10 rate. Amorphous TiO2 when prepared in ethanol and heated under vacuum showed an even better capacity retention of 240 mAh/g. From Fourier transform infra-red spectroscopy and electron energy loss spectroscopy measurements, the improved capacity is attributed to the complete removal of ethanol and the presence of very small fractions of residual functional groups coordinated to oxygen-deficient surface titanium sites. These displace the more reactive chemisorbed hydroxyl groups, limiting reaction with components from the electrolyte and possibly enhancing the integrity of the solid electrolyte interface. The present research provides a facile strategy to improve the capacity retention of nano-structured electrode materials.
Highlights
For approximately two decades graphite, allowing Li intercalation between the graphitic sheets, has been the dominant negative electrode for lithium ion batteries
We suggest that the better capacity retention of samples annealed at higher temperatures is due to a more stable solid electrolyte interface (SEI), which is the consequence of the absence of residual solvent molecules, unlike for samples annealed at 80 or 100°C, where lithium continues to react with reactive solvent species leading to a much lower capacity retention
To conclude, it has been shown that extended annealing under vacuum leads to capacity retention of 220 and 240 mAh/g after 50 cycles for amorphous TiO2 prepared in acetone and ethanol, respectively
Summary
For approximately two decades graphite, allowing Li intercalation between the graphitic sheets, has been the dominant negative electrode for lithium ion batteries. Among the most promising polymorphs are nanoparticles of TiO2-anatase (Sudant et al, 2005; Guo et al, 2007; Ren et al, 2010; Shin et al, 2011), nanowires and nanotubes of TiO2(B) (Armstrong et al, 2005a; Zukalova et al, 2005), and the amorphous TiO2 (Borghols et al, 2010), but all of them suffer from an irreversible loss of capacity during the first battery cycles and a capacity retention that is lower than the theoretical maximum. Recent attempts to diminish this initial capacity loss by using pre-lithiation to displace surface chemisorbed −OH groups by lithium appears to have been successful for TiO2-(B) (Brutti et al, 2012) nanotubes, resulting in a more stable solid electrolyte interface (SEI) and better capacity retention, though applying this treatment on a larger scale may prove more challenging
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