Combining experiments and computations to understand the intercalation potential and redox mechanism for A 2 Ti 3 O 7 (
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Combining experiments and computations to understand the intercalation potential and redox mechanism for A2Ti3O7 (A=Li, Na)
A. Morales-García,1 M. Elena Arroyo-de Dompablo,2 A. G. Rousse,3,4 P. Senguttuvan,5,6 J.-M. Tarascon,3,4 M. Rosa Palacín6 1
Department of Physical and Macromolecular Chemistry, Faculty of Science, Charles University in Prague, Hlavova 2030, Prague 2, 128 43, Prague (Czech Republic). 2 Departamento de Química Inorgánica, Universidad Complutense de Madrid, Avda Complutense sn, 28040 Madrid (Spain). 3 Collège de France, Chimie du solide et de l’Energie, FRE3677, 11 place Marcelin Berthelot, 75231 Paris Cedex 05 (France). 4 Sorbonne Universités-Université Pierre et Marie Curie UPMC Univ Paris 06, 4 Place Jussieu, 75252 Paris Cedex 05 (France). 5 Laboratoire de Réactivité et Chimie des Solides, UPJV, CNRS UMR6007, 33 rue Saint Leu 80039 Amiens (France). 6 Institut de Ciència de Materials de Barcelona (ICMAB-CSIC) Campus UAB, E-08193 Bellaterra, Catalonia (Spain).
ABSTRACT Na2Ti3O7, a potential negative electrode for Na batteries, is investigated by combining experiments and first-principles calculations at the Density Functional Theory (DFT) level. A structural model is proposed for the reduced phases (A2+xTi3O7), with all alkali ions in octahedral coordination, leading to a distorted rocksalt type structure. The calculated elastic constants support the mechanical stability of the proposed Na4Ti3O7 structure. Calculated average intercalation potentials are 0.37 V for Na insertion in Na2Ti3O7 and 1.46 V for Li insertion in Li2Ti3O7, being in very good agreement with the values observed experimentally (0.3 V and 1.6 V respectively). The higher polarizing character of Li ions vs Na ions acts as a key-factor to bring the Li intercalation voltage 0.7 V above that of Na intercalation in layered-A2Ti3O7 materials. INTRODUCTION The development of room temperature sodium based batteries is currently a challenge in fundamental materials research. Along this line, we recently reported on Na2Ti3O7, a well-known oxide previously studied for a wide range of applications, which turned out to reversibly uptake 2 Na ions per formula unit (200 mAh/g) at an average potential of 0.3 V vs Na+/Na0. [1] To our knowledge, this is the first ever reported oxide to reversibly react with sodium at such a low potential, which could tentatively be coupled to any developed high potential positive electrode material to build high energy density Na-ion cells. These preliminary results were inspiring and prompted to optimism in terms of technology.
The structure of Na2Ti3O7 (S.G. P21m) is built upon TiO6 octahedra linked by edges, so as to form zig-zag 3 x 2 x ∞ ribbons (see Figure 1). These ribbons are connected by vertices and form a layered framework. Within this framework, Na ions are distributed among two distinct crystallographic sites, Na1 and Na2, which are coordinated by 9 and 7 oxygen atoms, respectively. Additional sites are available for the insertion of alkali ions. By ion exchange from Na2Ti3O7, it is possible to
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