Research in the area of solids open-framework metal phosphates, has been experiencing considerable growth over the past two decades. Since 1980s, a series of transition metal phosphates with three-dimensional structure have been investigated. More recently, it has been revealed that the phosphite group, could also be of great interest, because of its tendency to form new solids open framework structure.

In the literature, mixed-phosphite metal compounds have been widely synthesized with a variety of cations going from alkali metals to group IIIA of the periodic table. Some rare earth based phosphites have been also isolated. However, to the best of our knowledge, no compound with titanium and  phosphite group has been reported. This compound has an interesting structure allowing it to be used as insertion material for electrodes of Li-ion batteries, while only phosphates and phosphides compounds have been used as electrode materials.    

Recently, two inorganic phosphite materials were tested as electrodes for energy storage systems and have shown very small specific capacities toward lithium insertion [1,2]. Besides, both works dealt with the use of phosphite based-compound as positive electrode.

In this study, we are presenting a new titanium phosphite compound used as negative electrode material for lithium ion batteries. Ti₂(HPO₃)₃ has been prepared under hydrothermal conditions and autogenous pressure. The structure of titanium phosphite with the determined crystallographic formula Ti₂(HPO₃)₃ was solved in hexagonal system in centrosymmetric P6₃/m space group from single crystal diffraction data. The structure was solved by the direct method using, refined by full matrix least square. It has a three dimensional structure with tunnels along a- and b-axes (see figure). This is exactly what interests us to test it as an insertion electrode material.

This compound displays a high thermal stability limit showed by TGA analysis. Structural and chemical characterization have been realized by XRD, IR and Raman spectroscopies respectively. Electrochemical analyses were done by galvanostatic cycling and cyclic voltammetry. On the initial discharge of cell, the Ti₂(HPO₃)₃ electrode delivers a capacity of 740 mAh/g.

In the aim of enhancing the electrochemical performances, Ti₂(HPO₃)₃/graphene composite was also prepared via hydrothermal route. Results of both compounds will be presented and compared in this present publication.

[1] U. Chung, J.L. Mesa, J.L. Pizarro, I. de Meatza, M. Bengoechea, J. Rodríguez Fernandez, M. I. Arriortua, and T. Rojo, J. Chem. Mater. 23 (2011) 4317-4330.

[2] H. Yaghoobnejad Asl, K. Ghosh, M. P. Vidal Meza, A. Choudhurya, J. The Royal Society of Chemistry. (2013),  00, 1-3.