DOI: 10.1002/chem.201302306 Ammonia-Annealed TiO 2 as a Negative Electrode Material in Li-Ion Batteries: N Doping or Oxygen ACHTUNGTRENNUNGDeficiency? Edgar Ventosa,* [a] Wei Xia, [b] Stefan Klink, [a] Fabio La Mantia, [c] Bastian Mei, [b] Martin Muhler, [b] and Wolfgang Schuhmann [a, c] Introduction Energy storage is a key element in achieving sustainable and environmentally friendly energy systems. Li-ion batter- ies (LIBs) have been extensively studied due to their high energy and power density. [1, 2] Among all the promising nega- tive electrode materials for LIBs, titanium dioxide (TiO 2 ) has recently attracted much attention. [3, 4] Despite the lower energy density of LIBs based on TiO 2 due to its high operat- ing potential (  1.8 V vs. Li/Li + ), TiO 2 presents several ben- efits compared with traditional graphite, such as avoiding the formation of the solid electrolyte interphase (SEI) and increased safety, especially at high rates and elevated tem- peratures. [5] Furthermore, TiO 2 is abundant and cheap, and it shows low self-discharge, good cyclability and excellent C- rate retention. [6–9] The performance of TiO 2 electrodes for Li-ion batteries, however, is limited by 1) the sluggish diffusivity of Li-ions and 2) the poor electric conductivity. The former has been addressed by shortening the length of the Li-ion diffusion pathway through nanostructuring [2, 10] and mesoporosity, [6, 11] by controlling the particle shape to favour one diffusion di- rection [12] or by controlling the crystalline phase to enhance Li-ion mobility. [13, 14] Electric conductivity has been improved by coating TiO 2 with carbon [15] or RuO 2 , [16] by wiring aggre- gates of TiO 2 with carbon nanotubes [17] or by doping TiO 2 with native [18] or foreign [19] elements. Ammonia annealing, which has been extensively used to prepare N-doped TiO 2 in photocatalysis, [20, 21] has been shown to improve the performance of lithium titanate (Li 4 Ti 5 O 12 ) as negative electrode materials in LIBs. [22] Such improvement was initially attributed to the formation of a TiN thin film at the surface, which increases the electric con- ductivity. [22] Recent findings seem to indicate that nitrogen doping of titanate is responsible instead of the formation of a TiN thin film. [23] In the case of anatase TiO 2 , the maximum annealing temperature is limited to about 600 8C, due to the phase transformation from anatase to the undesired rutile form starting at around that temperature. [24] Therefore, TiN thin films cannot be formed at the surface of anatase TiO 2 without changing the phase. However, the findings of Wan et al. [23] with respect to nitrogen doping of Li 4 Ti 5 O 12 open the possibility to improve the performance of anatase TiO 2 by ammonia annealing as well, since N doping does not re- quire such high temperatures. A systematic study of the effects of ammonia annealing on the electrochemical performance of anatase TiO 2 as neg- ative electrode, however, is needed. Since hydrogen anneal- ing has been shown to improve the performance of TiO 2 by creating oxygen vacancies that increase the electric conduc- tivity, [18] and taking into consideration that ammonia anneal- ing could introduce nitrogen dopants as well as oxygen va- cancies, [25] the question arises whether nitrogen doping or the formation of oxygen vacancies are more relevant for the electrochemical performance of TiO 2 . Abstract: Improving the chemical dif- fusion of Li ions in anatase TiO 2 is es- sential to enhance its rate capability as a negative electrode for Li-ion batter- ies. Ammonia annealing has been used to improve the rate capability of Li 4 Ti 5 O 12 . Similarly, ammonia anneal- ing improves the Li-ion storage per- formance of anatase TiO 2 in terms of the stability upon cycling and the C- rate capability. In order to distinguish whether N doping or oxygen deficien- cies, both introduced upon ammonia annealing, are more relevant for the observed improvement, a systematic electrochemical study was performed. The results suggest that the creation of oxygen vacancies upon ammonia an- nealing is the main reason for the im- provement of the stability and C-rate capability. Keywords: high C rate · lithium-ion battery · nitrogen dopant · oxygen deficiency · titanium dioxide [a] Dr. E. Ventosa, Dr. S. Klink, Prof. W. Schuhmann Analytische Chemie - Elektroanalytik & Sensorik Ruhr-Universität Bochum Universitätsstrasse 150, 44780 Bochum (Germany) Fax: (+ 49) 2343214683 E-mail: edgar.ventosa@rub.de [b] Dr. W. Xia, Dr. B. Mei, Prof. M. Muhler Laboratory of Industrial Chemistry Ruhr-University Bochum Universitätsstrasse 150, 44780 Bochum (Germany) [c] Dr. F.L. Mantia, Prof. W. Schuhmann Center for Electrochemical Sciences - CES Ruhr-Universität Bochum Universitätsstrasse 150, 44780 Bochum (Germany) Chem. Eur. J. 2013, 00,0–0  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! ÞÞ &1& FULL PAPER