The Kinetics of Nanometer Sized TiO2 Films as a Lithium-Ion Insertion Electrode

The miniaturization of autonomous wireless sensors, implantable devices and energy harvesters, has steadily been increasing in the last decades. However, the pace of miniaturization of batteries has been lagging behind, with typical sizes of batteries two to a thousand times larger than the corresponding microelectronic device¹.

To solve this problem, planar all-solid-state thin-film Li-ion micro-batteries have been developed and commercialized. Although they offer size reduction and a significant increase in power density compared to conventional rechargeable coin cells, the limitations due to diffusion resistances and mechanical integrity of such planar devices limits the maximum thickness of the active electrodes and consequently the capacity. To increase the energy density, ultra-thin films could be deposited on three-dimensional substrates thereby significantly increasing the power density and capacity, while preventing detrimental effects such as film cracking and delamination. For this, deposition techniques that can deposit conformably and pinhole-free on 3D structures is necessary. Atomic layer deposition (ALD) is therefore an interesting deposition technique, since structures with very high aspect ratios (~5000) can be coated².

However, to successfully design 3D (micro-)batteries, the properties of ultra-thin films have to be investigated. A material that shows high capacity (~200 mAh/g), especially upon nanostructuring, is TiO₂. Such nanostructuring strategies include using nanoparticles³,  nanotubes⁴ or nanoflakes⁵, but the characterization of a well-defined system, such as thin TiO₂ films with the same length scale (5 - 35 nm), is still lacking. Therefore, in this work we electrochemically characterize TiO₂ thin-films of 5 nm to 35 nm deposited with atomic layer deposition (ALD). Structure and morphology of the films were examined by transmission electron microscopy (TEM) and grazing-incidence X-Ray diffraction (GI-XRD), and showed that the crystal structure of as-deposited TiO₂ changes from amorphous TiO₂ for the 5 nm TiO₂ films to anatase for the 35 nm TiO₂films.

Electrochemical characterization was done using cyclic voltammetry and galvanostatic charge/discharge. Cyclic voltammetry results shown in figure 1a clearly reflected the difference between lithium insertion/extraction into amorphous and anatase TiO₂. Furthermore, scan rate dependent peak current measurements showed how the lithium diffusion behavior changed from semi-infinite diffusion for the 35 nm films to finite-length diffusion for the 5 nm films.

The improved kinetics for thinner films were also captured by galvanostatic charge/discharge measurements. Figure 1b shows the volumetric capacity of lithiation; at a charging rate of 1C, 5 nm TiO₂ had a reversible volumetric capacity of ~150 µAh cm^-2 µm^-1, which is even above the theoretical capacity of TiO₂ (~142 µAh cm^-2 µm^-1). Comparatively, 35 nm TiO₂ only achieved a capacity of ~50 µAh cm^-2 µm^-1. Upon increasing the charging rate to 200C (full theoretical charging in 18 seconds), 50% of the original capacity was retained (~75 µAh cm^-2 µm^-1) for 5 nm TiO₂ and only 10% of the original capacity was retained for 35 nm TiO₂ (~5 µAh cm^-2 µm^-1).

These results highlight the great potential of TiO₂thin-films as an electrode in 3D (micro-)batteries and are a good example how simply scaling of the electrode thickness can significantly enhance the volumetric capacity and rate-performance.

References
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5. M.-C. Yang, Y.-Y. Lee, B. Xu, K. Powers, and Y. S. Meng, J. Power Sources, 207, 166–172 (2012)