Aug 22, 2013 - of control over the 3D electrode design, is relatively cheap and easy to scale up. 1. INTRODUCTION ... sources such as wind and solar power.
Article pubs.acs.org/JPCC
Nanostructured TiO2 Anatase Micropatterned Three-Dimensional Electrodes for High-Performance Li-Ion Batteries Deepak P. Singh,† A. George,‡ R.V. Kumar,§ J.E. ten Elshof,‡ and Marnix Wagemaker*,† †
Department of Radiation Science and Technology, Delft University of Technology, Mekelweg 15, Delft 2629JB, The Netherlands MESA+ Institute of Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands § Department of Material Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, United Kingdom ‡
S Supporting Information *
ABSTRACT: Soft lithography using polydimethylsilicate molds is shown to be a novel promising method to prepare three-dimensional (3D) micro Li-ion electrodes, demonstrated by the synthesis of nanometer 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 for the optimization of ionic and electronic wiring. The resulting 3D microshaped 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 highperformance electrode preparation which, in addition to providing a large degree of control over the 3D electrode design, is relatively cheap and easy to scale up.
electrode materials, including rutile,3 anatase,4−6 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 because of its 3D Li-ion diffusion pathway. The main approach for improving the electronic and ionic charge transport of Li-ion electrodes is adding conductive additives such as carbon and reducing the TiO2 particle size, respectively.3,9−13 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 reaction.11,16 In addition, nanosizing 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 profiles.20,21 Whether the favorable properties of nanomaterials can actually be utilized depends on the charge transport through 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
1. INTRODUCTION Efficient energy storage on all scales is becoming increasingly important in our society. Efficient electricity storage on kWh to MWh scale is a prerequisite for the energy transition from fossil fuels toward 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 because of their 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 a wide variety of three-dimensional (3D) electrode architectures.1,2 Within the large group of Li-ion insertion hosts, TiO2 is a promising class of electrode materials for Li-ion batteries because of its excellent volumetric and gravimetric storage capacities and because it is environmentally benign, nontoxic, highly abundant, and relatively cheap. Although the higher operating voltage, compared to that of the most common negative electrode, 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 compared to graphite. A variety of TiO2 polymorphs have been studied as © 2013 American Chemical Society
Received: December 3, 2012 Revised: August 21, 2013 Published: August 22, 2013 19809
dx.doi.org/10.1021/jp3118659 | J. Phys. Chem. C 2013, 117, 19809−19815
The Journal of Physical Chemistry C
Article
(ethylene carbonate and dimethyl carbonate in a 1:1 mass ratio) (Novolyte, battery grade). Because high charge rates are more relevant than discharge rates in practice, the cells were tested at a variable charge rate between 10C and 60C and always discharged with a C/2 or 1C rate within the voltage window between 3 and 0.9 V versus Li/Li+ using a Maccor 4300 battery cycler. The electrode morphologies and microstructure were studied by a JEOL 7500F scanning electron microscope. The crystal structure of the active material was characterized by X’Pert PRO X-ray diffractometer (PANalytical) applying Cu Kα radiation.
and the AM. Because of all these aspects, 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 microstructure,1,2,22 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/wires,23,24 architecture nano sheets,25 core−shell structures,26 graphene− TiO2 composites,27 and nano microspheres,28−30 or direct templating of metal nanowires,30−33 aligned carbon foam,34 and peptide 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 at this stage is limited to relatively thin electrodes with low aspect ratios, we foresee that the advances in this field will allow electrode preparation beyond the field of microelectrodes toward the conventional Li-ion battery electrodes.
3. RESULTS AND DISCUSSION Figure 1a shows schematically the procedure for fabricating 3D micropatterns of nanosized anatase TiO2 electrodes using
2. EXPERIMENTAL SECTION 2.1. Fabrication of Polydimethylsilicate (PDMS) Templates. The micrometer 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). Polydimethylsiloxane (PDMS) polymer and curing agent (Sylgard 184) were mixed in a 10:1 mass ratio and polymerized on the patterned silicon wafer (precoated with a 1H,1H,2H,2Hperfluorooctylsilane monolayer as antiadhesion layer). The PDMS was cured at a temperature of 70 °C 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 g of triblock copolymer Pluronic P123 in 12 g of absolute ethanol. The P123/TTIP molar ratio was adjusted to 0.05. The mixture was then magnetically stirred after the addition of 10 mL of deionized water. After being stirred for an additional 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). After the PDMS mold was pressed on the deposited film, the deposited films were aged at 60 °C for 72 h. The resulting sol was converted to a gel by hydrolytic condensation, which was followed by calcination at 150−200 °C and 400 °C for 2 h to remove the P123 triblock copolymer template and produce the TiO2 anatase electrode. 2.3. Battery Preparation and Electrode Characterization. Battery cells were assembled in Swagelok type cells under argon atmosphere (
