Lithium Storage in Amorphous TiO[sub 2] Nanoparticles

Wouter J. H. Borghols, Dirk Lützenkirchen-Hecht, Ullrich Haake, Wingkee Chan, Ugo Lafont, Erik M. Kelder, Ernst R. H. van Eck, Arno P. M. Kentgens, Fokko M. Mulder, Marnix Wagemaker
2010 Journal of the Electrochemical Society  
Amorphous titanium oxide nanoparticles were prepared from titanium isopropoxide. In situ measurements reveal an extraordinary high capacity of 810 mAh/g on the first discharge. Upon cycling at a charge/discharge rate of 33.5 mA/g, this capacity gradually decreases to 200 mAh/g after 50 cycles. The origin of this fading was investigated using X-ray absorption spectroscopy and solid-state nuclear magnetic resonance. These measurements reveal that a large fraction of the total amount of the
more » ... d Li atoms is due to the reaction of H 2 O/OH species adsorbed at the surface to Li 2 O, explaining the irreversible capacity loss. The reversible capacity of the bulk, leading to the Li 0.5 TiO 2 composition, does not explain the relatively large reversible capacity, implying that part of Li 2 O at the TiO 2 surface may be reversible. The high reversible capacity, also at large ͑dis͒charge rates up to 3.35 A/g ͑10C͒, makes this amorphous titanium oxide material suitable as a low cost electrode material in a high power battery. Electrochemical storage devices based upon lithium-ion technology have replaced earlier battery types in numerous applications, e.g., portable devices, mainly due to their high energy density, long cycle life, and their relatively low impact on the environment. If materials that support higher current densities during discharging and satisfy the safety issues concerned, Li-ion batteries would become available for heavy duty applications such as ͑hybrid͒ electrical cars. A high power density requires both good ionic and electronic transport properties of the electrode materials. In many cases, the solid-state diffusion of Li ions through the electrode materials is several orders of magnitude smaller than in the electrolyte. Therefore, if the power density is to be improved, the electrode performance is to be investigated. In commercially available Li-ion batteries, the electrode material is dispersed in the electrolyte as microsized crystallites, which are capable of hosting the lithium ions inside their crystalline voids. By simply decreasing the size of these crystallites, the electrode-electrolyte interface is increased, whereas the diffusion length inside the electrode crystallite decreases. However, recent studies reveal a more complex behavior of nanosized Li insertion compounds in, e.g., TiO 2 anatase, 1-3 TiO 2 rutile, 4,5 or Li x FePO 4 , 6 showing distinct changes in electronic structure and ionic mobility upon downsizing to the nanodomain. 7 Usually, these differences in electronic structure and ionic mobility between bulk and nanosized crystallites are ascribed to the relatively increased impact of surface phenomena. 8-10 Between the crystalline structures anatase and rutile TiO 2 , similarities were observed in the physical behavior of the nanoscale compounds. Both reveal an increased Liion capacity compared to their microscale counterparts, which appears to be facilitated by an anomalous phase behavior that is induced by the nanoscale. 2,5 The enhanced Li capacity of both structures is confirmed by electrochemical experiments, and, in addition, these studies suggest a decrease in Li-ion mobility because the capacity decreases as the dis͑charge͒ rate increases. 4, 11 Several other polymorphs of TiO 2 are also reported to store Li ions, such as hollandite, 8 ramsdellite, 9 TiO 2 ͑B͒, 10 and amorphous TiO 2 , 11 but the most abundant morphologies of anatase and rutile are the most widely studied materials. Here, we explore the lithium insertion and extraction in nanoscale amorphous TiO 2 . Aiming at a complete understanding, we have, in addition to standard electrochemical ͑dis͒charging, applied several microscopic and macroscopic probes including neutron diffraction ͑ND͒, X-ray absorption spectroscopy ͑XAS͒, 7 Li nuclear magnetic resonance ͑NMR͒, energy-dispersive spectroscopy ͑EDS͒, and thermogravimetric analysis ͑TGA͒. This has led to a consistent picture of the performance of this material revealing amorphous TiO 2 as a very promising candidate for cheap, high power, and high capacity anodes in Li-ion batteries. Methods Sample preparation.-Titanium isopropoxide, obtained from Aldrich, was mixed with demineralized water. The resulting white precipitate was filtered and subsequently washed with ethanol several times. This material is referred to as pristine amorphous TiO 2 ͑PA-TiO 2 ͒. This PA-TiO 2 is left to dry inside a vacuum oven at 400 K for a week. This vacuum-dried TiO 2 is referred to as VD-TiO 2 . The chemically lithiated amorphous sample was prepared by first dispersing the VD-TiO 2 in hexane ͑anhydrous 95 + %, Aldrich͒, after which an excess n-butyllithium ͑1.6 M Aldrich͒ 12,13 was slowly added while stirring the mixture. This method allows insertion of lithium in a chemical way by an electrochemical driving force. To avoid lithium from reacting directly with air and moisture, the procedure was performed in a glove box under argon atmosphere having less than 1 ppm O 2 and moisture, both responsible for the formation of Li 2 O, LiOH, Li carbonates, and Li nitrates 14 as potential impurities. The resulting Li mole fraction inside the amorphous host was determined by wet chemical inductively coupled plasma ͑ICP͒ spectroscopy, and the atomic percentages of Ti and O were obtained by EDS during transmission electron microscopy ͑TEM͒ measurements ͑Philips CM30T͒. The TGA data were recorded using a Perkin-Elmer TGA 7 thermogravimetric analyzer. Neutron Diffraction.-The room temperature ND measurements were performed at POLARIS and the medium resolution, high intensity time-of-flight diffractometer at the ISIS pulsed neutron source ͑Rutherford Appleton Laboratory, U.K.͒. POLARIS is equipped with 434 detectors in 4 banks, covering angles between approximately 160°͑backward scattering͒ and 13°͑forward scatter-ing͒. Both the pristine and lithiated samples were loaded in an argon atmosphere into airtight vanadium sample containers sealed with indium O-rings. The resulting ND patterns were refined using the z
doi:10.1149/1.3332806 fatcat:yuepurp2lfcvvfat45satsds3i