Highly conformal electrodeposition of copolymer electrolytes into titania nanotubes for 3D Li-ion batteries
© Plylahan et al.; licensee Springer. 2012
Received: 30 April 2012
Accepted: 12 June 2012
Published: 27 June 2012
The highly conformal electrodeposition of a copolymer electrolyte (PMMA-PEO) into self-organized titania nanotubes (TiO2nt) is reported. The morphological analysis carried out by scanning electron microscopy and transmission electron microscopy evidenced the formation of a 3D nanostructure consisting of a copolymer-embedded TiO2nt. The thickness of the copolymer layer can be accurately controlled by monitoring the electropolymerization parameters. X-ray photoelectron spectroscopy measurements confirmed that bis(trifluoromethanesulfone)imide salt was successfully incorporated into the copolymer electrolyte during the deposition process. These results are crucial to fabricate a 3D Li-ion power source at the micrometer scale using TiO2nt as the negative electrode.
Nowadays, microbatteries are in demand as power source to drive small devices such as smartcards, medical implants, sensors, etc. To date, the electrochemical performances of these all-solid-state batteries are limited because planar thin films are employed as electrode and electrolyte materials. In general, the total thickness of the stacking films is below 15 μm, and the resulting battery reveals relatively low power and energy densities. In order to ensure significant advances for extended applications, it is crucial to improve the electrochemical performances by investigating new materials and manufacturing processes. In this context, the large specific area offered by nano-architectured electrodes represents a promising alternative to improve the general performances of these micro-power sources .
Particularly, better rate capability, capacity, and cycling behavior have been observed for self-organized nanostructures such as titania nanotubes (TiO2nt) [2–9]. However, when targeting 3D microbatteries, the conventional top-down approach to deposit solid electrolyte (e.g., lithium phosphorous oxynitride) [10–12] is not really suitable due to the accumulation of the electrolyte at the top of the nanotubes . Certainly, with this accumulation of electrolyte, the 3D paradigm of microbatteries cannot be realized. Thus, investigating the deposition of polymer electrolytes into nanostructures by electrochemical techniques is a convenient way to ensure the desired filling of the nanostructures . Indeed, electropolymerization is particularly powerful to control the deposition of different polymers into various porous materials [15–19]. Very recently, the use of electrodeposition to fill TiO2nt with a layer of poly(methyl methacrylate)-polyethylene oxide, i.e., PMMA-(PEO)475 has been reported [13, 20]. We have demonstrated that this simple bottom-up approach is adequate to deposit a homogeneous copolymer layer into titania nanotubes while improving the electrochemical performance. However, conformal coating of the nanotubes by the polymer electrolyte is required to design a 3D microbattery. In this work, it is reported that the conformal deposition of a PMMA-PEO electrolyte into self-organized TiO2nt can be obtained by controlling the electrodeposition parameters. The morphology and the chemical composition of the resulting copolymer-embedded TiO2nt materials are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The incorporation of lithium bis(trifluoromethanesulfone)imide, so-called LiTFSI salt, into the electrodeposited polymer is studied by X-ray photoelectron spectroscopy (XPS).
Synthesis of self-organized TiO2nt has been widely reported for a wide range of applications [21–24]. In the present work, TiO2nt layers were produced by the electrochemical anodization of Ti foils using the Modulab potentiostat from Solartron Analytical (Hampshire, UK). Before the anodization, Ti foils with the 99.6+ % purity and 0.125-mm thickness were cut into pieces with the desired dimensions and sonicated sequentially during 10 min in acetone, propanol, and methanol sequentially. After that, the foils were rinsed with deionized water and dried with compressed air. The anodization process was carried out in an electrochemical cell containing a solution of 1 M H3PO4, 1 M NaOH, and 0.4 wt.% of HF. The setup consisted of Ti foil as the working electrode, a Pt grid as the counter electrode, and a Hg/Hg2SO4, K2SO4 (saturated) (E=0.64V vs NHE) reference electrode. A constant voltage of 20 V was applied during 2 h. The material was rinsed with deionized water and dried with compressed air immediately after the anodization process.
Then, an aqueous electrolyte containing 0.035 M LiTFSI was introduced into the cell and purged with N2 gas for 10 min before adding 4 g of the MMA-(PEO)475 monomer provided by Sigma Aldrich (St. Louis, MO, USA). It can be noticed that no initiator was added into the solution. The copolymer-embedded TiO2nt was obtained by cyclic voltammetry (CV) using the as-prepared TiO2nt layers as the working electrode and a Pt grid as the counter electrode. The CV curves were carried out in the potential window ranging from −0.4 to −2.5 V vs Hg/Hg2SO4, K2SO4 (saturated) with the scan rate of 25 mV/s. The number of cycles was varied from 1 to 10 in order to observe the influence of cycle number on the polymer electrolyte layers. After electropolymerization, the samples were dried at room temperature to evaporate part of the residual water. The morphology of the copolymer-embedded TiO2nt was studied by SEM and TEM analyses using a JEOL 6320 F SEM and a JEOL 2010 F TEM (JEOL Ltd., Akishima, Tokyo, Japan). XPS measurements were carried out with a Kratos Axis Ultra spectrometer (Kratos Analytical Ltd., Manchester, UK), using focused monochromated Al Kα radiation (hν = 1,486.6 eV). The XPS spectrometer was directly connected to an argon dry box through a transfer chamber to avoid moisture/air exposure of the samples. The analyzed area of each sample was 300 μm × 700 μm. Peaks were recorded with constant pass energy of 20 eV. The pressure in the analysis chamber was around 5 × 10−8 Pa. Short acquisition time spectra were recorded before and after each normal experiment to check that the samples did not suffer from degradation under the X-ray beam during measurements. Peak assignments were made with respect to experimental reference compounds, namely bulk anatase and/or rutile TiO2. The binding energy scale was calibrated from hydrocarbon contamination using the C 1 s peak at 285.0 eV. Core peaks were analyzed using a non-linear Shirley-type background. The peak positions and areas were optimized by a weighted least-square fitting method using 70% Gaussian and 30% Lorentzian line shapes. Quantification was performed on the basis of Scofield's relative sensitivity factors.
Results and discussion
Thickness of nanotube walls and inner diameters of tubes
Number of cycles
Thickness of nanotube walls (nm)
Inner diameter of nanotubes (nm)
The highly conformal electrodeposition of copolymer electrolyte has been successfully achieved on titania nanotubes. It is demonstrated that control of the electropolymerization parameters allows to homogeneously cover the nanostructures without closing the tubes. By this technique, the copolymer-embedded titania nanotubes retain the 3D structure which is advantageous for the further fabrication on high-performance 3D microbatteries.
We acknowledge the French Ministry of Education, C'Nano PACA, Région PACA, and ANR JCJC number 2010 910 01 for financial support. We would like to thank Serge Nitsche and Damien Chaudanson from the electron microscopy service of CINaM Laboratory (UMR 7325) for their assistance in obtaining SEM images.
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