A Structural and Electrochemical Study of Li 2 Ti 6 O 13
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A Structural and Electrochemical Study of Li2Ti6O13 J. C. Pérez-Flores, A. Kuhn and F. García-Alvarado Universidad San Pablo CEU, Departamento de Química. E-28668, Boadilla del Monte, Madrid, SPAIN
ABSTRACT The lithium titanate Li2Ti6O13 has been prepared from Na2Ti6O13 by Li ion exchange in molten LiNO3 at 325ºC. Chemical analysis and powder X-ray diffraction study of the reaction product, respectively, indicate that total Na/Li exchange takes place and the Ti-O framework of the Na2Ti6O13 parent structure is kept under those experimental conditions. The electrochemical characterization shows that Li2Ti6O13 is able to insert ca. 5 Li per formula unit under equilibrium conditions in the voltage range 1.5-1.0 V vs. Li+/Li. This corresponds to a specific discharge capacity of 250 mAh g-1. Lithium insertion occurs at an average equilibrium voltage of 1.5 V which is typical for oxides and titanates where Ti(IV)/Ti(III) is the active redox couple. After the first redox cycle a high reversible capacity is obtained (ca. 160 mAh g-1 at C/12, with a 70% capacity retention related to a phase transformation upon cycling). On the basis of these results, we are proposing Li2Ti6O13 as new lithium battery anode material to be further investigated. INTRODUCTION Lithium titanates such as ramsdellite-type Li2Ti3O7 [1], LiTi2O4 [2] or spinel-type Li4Ti5O12 [3] have been widely investigated as insertion electrode materials for rechargeable lithium ion batteries. They involve partial or full reduction of Ti4+ to Ti3+ ions at relatively low voltage (ca. 1.5 V) with a specific capacity value under dynamic conditions ranging from 150 to 175 mAh g−1. On the other hand, lithium titanates and titanium oxides suffer very low structural stress upon lithium insertion, characterized by a lack of dimensional change in the structure and the electrode conformation [1,3-5] which provide them with a long cycle life [6]. Size reduction to nanometric scale decreases the diffusion length to fully insert the grains at higher current density. These type of results encourages researchers in this field to prepare materials following chemical routes that may produce nanoparticles with controlled morphology or nanostructured electrodes [7,8]. A second series of alkali titanates corresponding to the chemical formula A2TinO2n+1 (A=alkali metal; n=3-9) can be of interest to develop new electrode materials because of the open structures with corrugated sheets of edge-and corner-sharing Ti-O6 octahedra. For A2Ti6O13 (e.g. n=6), the corrugated sheets are further corner-cross-linked, producing an open tunnel structure, whose interstitial sites are occupied by the alkali metal. In this work we have synthesized Li2Ti6O13 by ion exchange of Na+ by Li+ in Na2Ti6O13 [9,10]. The as-obtained lithium titanate is structurally characterized by Rietveld analysis of X-ray powder data. Furthermore, we have analyzed the electrochemical lithium insertion and deinsertion properties of Li2Ti6O13 to assess its possible use as negative electrode for rechargeable Li batteries.
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