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Nanostructured TiO Anatase Micro Patterned 3D Electrodes for High Performance Li-Ion Batteries Deepak P. Singh, Antony George, Ramachandran Vasant Kumar, Johan E. ten Elshof, and Marnix Wagemaker J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp3118659 • Publication Date (Web): 22 Aug 2013 Downloaded from http://pubs.acs.org on September 15, 2013
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The Journal of Physical Chemistry
Nanostructured TiO2 Anatase Micro Patterned 3D Electrodes for High Performance Li-ion Batteries Deepak P. Singh1, A. George2, R.V. Kumar3, J.E. ten Elshof2 and Marnix Wagemaker1* 1
Department of Radiation Science and Technology, Delft University of Technology, Mekelweg 15, Delft 2629JB, The Netherlands
2
MESA+ Institute of Nanotechnology, University of Twente, Enschede, The Netherlands
3
Department of Material Science and Metallurgy, University of Cambridge, Cambridge, United Kingdom *E-mail –
[email protected] Abstract Soft lithography using PDMS molds is shown to be a novel promising method to prepare 3D micro Li-ion electrodes, demonstrated by the synthesis of nano-meter TiO2 anatase in the molds via a TiO2 sol. With this approach the 3D electrode morphology can be controlled to a large degree which allows optimisation of the ionic and electronic wiring. The resulting 3D micro shaped nano-TiO2 electrodes, with no conducting or other additives, show very high rate performance in combination with high electrode densities (energy densities) and long cycle life. These results demonstrate the potential application of soft lithography techniques for high performance electrode preparation which, in addition to the large degree of control on the 3D electrode design, is relatively cheap and easy to scale up.
Keywords : Micro battery, Anatase, Soft lithography, Energy Density
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1.
Introduction:
Efficient energy storage is becoming increasingly important in our society on all scales. Efficient electricity storage on kWh to MWh scale is a prerequisite for the energy transition from fossil fuels towards electrification of mobility and renewable electricity sources such as wind and solar power. In addition, on the mWh to Wh scale high performance energy storage is necessary to keep up with the rapid progress and development of microelectronics and mobile electronic equipment. Electrochemical storage in Li-ion batteries is attractive having high energy and power densities and very high storage efficiencies typically exceeding 90%. Large energy densities require high specific lithium capacities of the electrodes and high potential difference between the electrodes whereas high power requires facile transport of the charged species, Li-ions and electrons. Improvement of charge transport can be achieved by tailoring the meso and nanostructure of the electrodes. In thin film micro batteries this has led to the development of wide variety of 3D electrode architectures1,2. Within the large group of Li-ion insertion hosts a promising class of electrode materials for Li-ion batteries is TiO2; in the first place because of its excellent volumetric and gravimetric storage capacities and secondly because TiO2 is environmentally benign, non-toxic, highly abundant and relatively cheap. Although the higher operating voltage, compared to for instance graphite, has the disadvantage of a lower battery voltage, the advantage is that it operates within the stability window of the normally applied organic electrolytes, making TiO2 inherently more stable and safe
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The Journal of Physical Chemistry
compared to the most common negative electrode graphite.
A variety of
TiO2 polymorphs have been studied as electrode materials including rutile3, anatase4-6, and TiO2 (B)7. The challenge is the low electronic conductivity of TiO2 and the low diffusivity of Li-ions in TiO2, which is governed by ambipolar transport. In particular the anatase polymorph appears to be suitable as electrode for both conventional4-6 as well as micro8 Li-ion batteries due to its 3D Li-ion diffusion pathway. The main approach to improve the electronic and ionic charge transport of Li-ion electrodes is to add conductive additives such as carbon and reducing the TiO2 particle size, respectively3,9-13. Nanosizing results in fundamental changes in the thermodynamics14,15 and often leads to higher storage capacities as demonstrated for anatase, reaching the maximum composition Li1TiO2 for 7 nm particles fully utilizing the Ti3+/4+ redox reaction11,16. In addition, nano-sizing leads to significant surface storage either at the solid-liquid or at the solid-solid interface resulting in capacities beyond the conventional bulk capacities17-19 and altered voltage profiles20,21. Whether the favourable properties of nano-materials can actually be utilized depends on the charge transport trough the complete electrode. This is governed by 1) electronic wiring (the contact between electrode composite and current collector, the conductive additive network and the contacts between the conductive network and the carbon coating of the active material (AM)), (2) the ionic network formed by the liquid electrolyte in the pores of the composite electrodes, and (3) the charge transfer reaction between the liquid electrolyte and the AM. Because of all these aspects,
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research is increasingly focusing on the complete morphology of the electrodes to improve battery performance. This has initiated development of various synthesis strategies that offer full control over the 3D electrode microstructure1,2,22, such that the electrolyte can easily penetrate into this network ensuring the fast ionic and electronic transport through the electrode. For TiO2 this resulted in the development of nanotubes/wires23,24, architecture
nano
composites27,
nano
nanowires30-33,
sheets25,
core-shell
microspheres28-30,
aligned
carbon
or
foam34
structures26, direct and
graphene-TiO2
templating
peptide
of
metal
arrays35
using
electrochemical and atomic layer deposition techniques. Although these morphologies improve the electronic and electrolyte wiring within the electrode matrix they generally result in low to very low tap densities (g/cm3), hence leading to low energy densities. In addition the synthesis methods are often time consuming and expensive. Here
we
report
on
self-supporting
three-dimensional
(3D)
micropatterned electrodes of pure (i.e. without conducting additives such as carbon) nanoporous TiO2 anatase showing excellent rate performances that compare to the best reported in literature, and having very high tap densities. The applied soft lithography technique36 is relatively cheap, easy to scale up and allows a large degree of freedom in the electrode design and applied active materials. Although soft lithography is limited to relatively thin electrodes with low aspect ratios at this stage we foresee that the advances in
this
field
will
allow
electrode
preparation
beyond
the
microelectrodes towards the conventional Li-ion battery electrodes.
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field
of
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2.
Experimental Section: 2.1
The
Fabrication of Polydimethylsilicate (PDMS) templates:
micrometre
scale
silicon
masters
were
patterned
by
standard
photolithography and etching to obtain a bas relief structure consisting of square arrays of cylindrical holes with a diameter of 3 µm and depth of 6 µm (Lionix BV, Netherlands). Polydimethylsiloxane (PDMS) polymer and curing agent (Sylgard 184) were mixed in a 10:1 mass ratio and polymerized on the
patterned
silicon
wafer
(pre
coated
with
a
1H,1H,2H,2H-
perfluorooctylsilane monolayer as anti-adhesion layer). The PDMS was cured at a temperature of 70 oC for 24 h. The resulting PDMS monoliths were a negative replica of the patterned silicon master. 2.2
Preparation of TiO2 Sol and Electrode Fabrication: To
prepare the TiO2 electrodes, titanium tetra isopropoxide (TTIP) was added to a solution of 1.0 gm of triblock copolymer Pluronic P-123 in 12 gm of absolute ethanol. The molar ratio P123/TTIP was adjusted to 0.05. The mixture was then magnetically stirred after adding 10 ml of deionised water. After stirring for another 3 h, the sol solution was deposited on metal substrate/current collector by spin-coating (400 rpm for 5 s, followed by 2000 rpm for 10 s). The deposited films were aged at 60oC for 72 h. The resulting sol converted to a gel by hydrolytic condensation, which was followed by calcination at 400 °C for 2 h to remove the P123 triblock copolymer template and produce the TiO2 anatase electrode. 2.3 Battery
Battery
cells were
Preparation
and
Electrode
assembled in Swagelok
type
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Characterization: cells under
argon
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atmosphere (
