Enhanced electrochemical performance of Lithium-ion batteries by conformal coating of polymer electrolyte
© Plylahan et al.; licensee Springer. 2014
Received: 16 May 2014
Accepted: 16 September 2014
Published: 2 October 2014
This work reports the conformal coating of poly(poly(ethylene glycol) methyl ether methacrylate) (P(MePEGMA)) polymer electrolyte on highly organized titania nanotubes (TiO2nts) fabricated by electrochemical anodization of Ti foil. The conformal coating was achieved by electropolymerization using cyclic voltammetry technique. The characterization of the polymer electrolyte by proton nuclear magnetic resonance (1H NMR) and size-exclusion chromatography (SEC) shows the formation of short polymer chains, mainly trimers. X-ray photoelectron spectroscopy (XPS) results confirm the presence of the polymer and LiTFSI salt. The galvanostatic tests at 1C show that the performance of the half cell against metallic Li foil is improved by 33% when TiO2nts are conformally coated with the polymer electrolyte.
KeywordsTitania nanotubes Polymer electrolyte Electropolymerization Lithium-ion batteries
The miniaturization of Lithium-ion batteries (LIBs) as a power source to drive small devices has been continuously developed to meet the market requirements of nomadic applications. The challenge of the miniaturization of LIBs is to minimize the size and, at the same time, maximize the energy and power densities of the battery. In this context, 3D Li-ion microbatteries have been developed to overcome this challenge. Particularly, nanoarchitectured electrodes such as self-organized titania nanotubes (TiO2nts) are a potential candidate as a negative electrode in 3D Li-ion microbatteries [1–9]. TiO2nts show a better electrochemical performance compared to the planar TiO2 counterpart  due to i) their high active surface area, ii) the direct contact between the active material and the substrate, iii) fast diffusion of charges, and iv) high stability upon cycling owing to spaces between nanotubes, which allow the volume variation caused by Li+ insertion/extraction. To achieve the fabrication of the full 3D Li-ion microbatteries, the use of conventional organic liquid electrolytes must be avoided due to the safety concerns and its incompatibility to the integrated circuit technology. Poly(ethylene oxide) or poly(ethylene glycol) (PEG) is the most common polymer electrolyte used in LIBs due to its compatibility to the all-solid-state Li-ion microbatteries , promising ionic conductivity [11, 12], and thermal stability . However, it is necessary to maintain the 3D nanotubular structure of TiO2nts after the deposition of the polymer electrolyte in order to subsequently fabricate the full 3D Li-ion microbatteries by filling the polymer-coated TiO2nts with a cathode material. Recently, we have reported the electropolymerization by cyclic voltammetry (CV) to conformally electrodeposit the PEG-based polymer electrolyte with bis(trifluoromethanesulfone)imide or LiTFSI salt on the TiO2nts without closing the tube opening [14–16]. The conformal electrodeposition of polymer is a convenient approach to preserve the 3D morphology of the substrate as it has been reported for other materials . The main advantage of the conformal coating is that all the active surface area of TiO2nts is in contact with the polymer electrolytes, which will improve the charge transport and thereby improving the performance of the microbatteries. It can be noted that the electrochemical-assisted polymerization process is a convenient and versatile approach. Indeed, the polymer formation can be achieved by different mechanisms involving the formation of free-radical intermediates, the activation by light, the use of initiator, and the direct oxidation or reduction of monomer as it has been reported for poly(para-phenylene)vinylene (PPV) , polypyrrole (PPy) , poly(2-methacryloyloxy(ethyl) acetoacetate) , and poly(sulfonated phenol) , respectively.
In this work, we report the fabrication of highly ordered and smooth TiO2nts and the conformal coating of the polymer electrolyte: comb-shaped poly(poly(ethylene glycol) methyl ether methacrylate) or P(MePEGMA) on TiO2nts by an electropolymerization technique. The morphology of the materials was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The chemical structure of the obtained P(MePEGMA) polymer was analyzed by proton nuclear magnetic resonance (1H NMR) and its molar mass was measured by size-exclusion chromatography (SEC). The polymer electrolyte was studied in depth by X-ray photoelectron spectroscopy (XPS). The electrochemical tests were carried out in the half cell to investigate the improved performance of the solid-state batteries caused by the conformal coating of TiO2nts with the polymer electrolyte.
TiO2nts were synthesized by electrochemical anodization  of Ti foil in an electrolyte containing 96.7 wt% glycerol, 1.3 wt% NH4F, and 2 wt% water. A constant voltage of 60 V was applied to the cell (Ti foil as a working electrode and Pt foil as a counter electrode) for 3 h. After the anodization, the sample was rinsed with deionized water without removing it from the cell.
The electropolymerization on as-formed TiO2nts was carried out by CV [14–16] in an aqueous electrolyte containing 0.5 M LiTFSI and 0.2 M poly(ethylene glycol) methyl ether methacrylate or MePEGMA in a three-electrode system: TiO2nts as a working electrode, Pt foil as a counter electrode, and Ag/AgCl, 3 M KCl as a reference electrode. MePEGMA is used as a starting monomer with an average molecular weight of 300 g mol-1 and an average repeating ethylene oxide unit of 5. Prior to the electropolymerization, the electrolyte was purged with N2 for 10 min to remove dissolved oxygen. The CV was done for five cycles in the three-electrode configuration at the scan rate of 10 mV/s, potential window of -0.35 to -1 V vs Ag/AgCl, 3 M KCl. After the electropolymerization, the sample was dried without rinsing under vacuum at 60°C overnight.
The morphology of TiO2nts and polymer-coated TiO2nts were examined by electron microscopy techniques using a JEOL 6320 F SEM and a JEOL 2010 F TEM (JEOL Ltd., Akishima, Tokyo, Japan).
1H NMR spectra in CDCl3 were recorded on a Bruker Advance 400 spectrometer (Bruker AXS, Inc., Madison, WI, USA). A chemical shift was given in parts per million relative to tetramethylsilane. In order to characterize the polymer by NMR, the sample was immersed during approximately 1 h in CDCl3. Then the solution was recovered and concentrated to reach the minimum volume for an NMR analysis (≈0.5 mL).
Polymer molecular weights and dispersities were determined by SEC. The used system was an EcoSEC (Tosoh, Tokyo, Japan) equipped with a PL Resipore Precolumn (4.6 × 50 mm, Agilent Technologies, Inc., Santa Clara, CA, USA) and two linear M columns (4.6 × 250 mm, Agilent) with a gel particle diameter of 3 μm. These columns were thermostated at 40°C. Detection was made by an UV/visible detector operated at λ = 254 nm, a dual flow differential refractive index detector, both from Tosoh, and a viscometer ETA2010 from PSS (Polymer Standards Service Inc., Amherst, MA, USA). Measurements were performed in THF at a flow rate of 0.3 mL min-1. Calibration was based on polystyrene standards from Polymer Laboratories (ranging from 370 to 371,100 g mol-1).
XPS measurements were performed on a Thermo K-alpha spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) with a hemispherical analyzer and a microfocused monochromatized radiation (Al Kα, 1,486.7 eV) operating at 72 W under a residual pressure of 1 × 10-9 mbar. Peaks were recorded with a constant pass energy of 20 eV. The spectrometer was calibrated using the photoemission lines of gold (Au 4f7/2 = 83.9 eV, with reference to the Fermi level) and copper (Cu 2p3/2 = 932.5 eV). The Au 4f7/2 full width at a half maximum (FWHM) was 0.86 eV. All samples were fixed on the sample holders in a glove box directly connected to the introduction chamber of the spectrometer to avoid moisture/air exposure of the samples. Charge effects were compensated by the use of a charge neutralization system (low-energy electrons) which has the unique ability to provide consistent charge compensation. Short acquisition time spectra were recorded before and after each experiment and compared, to check the non-degradation of the samples under the X-ray beam. The binding energy (BE) scale was calibrated from the hydrocarbon contamination using the C 1 s peak at 285.0 eV. Core peaks were analyzed using a linear background, except for the S 2p core peak for which a non-linear Shirley-type background was applied . The peak positions and areas were obtained by a weighted least-square fitting of model curves (using 70% Gaussian and 30% Lorentzian line shapes) to the experimental data. Quantification was performed on the basis of Scofield's relative sensitivity factors .
The electrochemical tests of bare TiO2nts and polymer-coated TiO2nts were carried out using galvanostatic measurements. TiO2nt electrodes were assembled against a metallic Li foil using Swagelok cells in an Ar-filled glove box. Two sheets of Whatman paper soaked with MePEGMA + LiTFSI electrolyte were used as a separator. The separators were prepared by soaking the circular disk (diameter of 10 mm) with an aqueous solution of 0.5 M LiTFSI + 0.8 M MePEGMA, then dried at 60°C in a vacuum dryer overnight. The cells were cycled at 1C in the potential window of 1.4 to 3 V vs Li/Li+ using VMP3 (Biologic instrument).
Results and discussion
Binding energies (BE, eV), full width at a half maximum (FWHM, %), and atomic percentages (At.%) of the main components of TiO 2 nts coated with P(MePEGMA) and TiO 2 nts coated with P(MePEGMA) + LiTFSI
TiO2nts + P(MePEGMA)
TiO2nts + P(MePEGMA) + LiTFSI
C 1 s
C 1 s
O 1 s
O 1 s
F 1 s
F 1 s
Li 1 s
N 1 s
To summarize, our results show that self-organized TiO2nts are successfully coated with the polymer electrolyte, and the presence of PEG and LiTFSI is confirmed.
The highly conformal electrodeposition of P(MePEGMA) polymer electrolyte on highly organized TiO2nts has been achieved using the cyclic voltammetry technique. The characterization of the polymer electrolyte by 1H NMR and SEC shows the formation of short polymer chains, mainly trimers and some traces of high molar mass polymer species as well as MePEGMA monomers. The in depth studies of polymer-coated TiO2nts by XPS confirm the presence of polymer and LiTFSI salt. The electrochemical tests of TiO2nts in the half cell against metallic Li foil show the improved performance of the LIBs by 33% at 1C when TiO2nts are conformally coated with the polymer electrolyte.
We acknowledge the Région PACA and ANR JCJC no. 2010 910 01 for financial supports.
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