Structural changes upon lithium insertion in Ni0.5TiOPO4

Graphical abstract: Ex situ X-ray diffraction patterns for the chemical lithium insertion in, Ni0.5TiOPO4:Lix phases as a function of Li amount (x). The results show formation of two phases (I) and (II) during the process. Highlights: ► Nickel titanium oxyphosphate Ni0.5TiOPO4 (NTP), was prepared by solid state reaction. ► The NTP electrode delivered a capacity of 530 mAh/g, upon cycling within 0.8–4 V. ► In situ synchrotron X-ray diffraction was achieved to elucidate the electrochemical reaction mechanism. ► High capacity equivalent to the intercalation of more than 3.5 lithium ions per Ni0.5TiOPO4. - Abstract: Nickel titanium oxyphosphate Ni0.5TiOPO4 (NTP), was prepared by co-precipitation route. Its structure was determined by single crystal X-ray diffraction. The compound crystallizes in the monoclinic system, S.G: P21/c [a = 7.333(1) Å, b = 7.316(2) Å, c = 7.339(2) Å, β = 119.62(3)°, Z = 4, R1 = 0.0142, wR2 = 0.0429]. The structure might be described as a {TiOPO4} framework made of corner-sharing [TiO6] octahedra chains running parallel to [0 0 1] and cross linked by phosphate [PO4] tetrahedral, where half of octahedral cavities created are occupied by Ni atoms, and the other half of octahedral sites are vacant. During the first discharge, the NTP electrode delivered a capacity of 530 mAh/g, upon cycling within 0.5–4 V. To understand the electrochemical reaction mechanism using different characterization techniques viz. in situ synchrotron diffraction. Reciprocal magnetic susceptibility (χ−1) of NTP, between 4 and 300 K, shows an almost linear behavior and can be fitted by the simple Curie–Weiss law.