Microstructural details of spindle-like lithium titanium phosphate revealed in three dimensions

Abstract

Lithium titanium phosphate LiTi2(PO4)3 is an electrode material for lithium-ion batteries with a specific capacity of 138 mA h g-1. Owing to its potential of 2.5 V vs. Li/Li+ it provides an electrochemically stable interface when used as an anode in all-solid state batteries with NASICON type lithium aluminium titanium phosphate electrolyte. High performance has been identified for in situ carbon coated LiTi2(PO4)3 synthesized via a hydrothermal route, resulting in micro-scaled spindle shaped particles consisting of nano-scaled sub-particles. To elucidate the internal microstructure of these spindle-like particles in three dimensions we applied tomographic Focused Ion Beam-Scanning Electron Microscopy. For more detailed chemical analysis we performed electron-energy loss spectroscopy and energy dispersive X-ray spectroscopy in the scanning electron microscope as well as high resolution (scanning) transmission electron microscopy for structural insight. It could be clearly shown that the spindle-like particles mainly are made up of LiTi2(PO4)3 sub-particles in the 100 to 400 nm range. Additionally, two types of secondary phase materials were identified. LiTiOPO4, which shows different surface morphology, as a volume component of the spindles and TiO2 nanoparticles (anatase), which are not only present at the particle surface but also inside the spindle, were detected. Reconstruction from tomography reveals the nanoparticles form a three-dimensionally interconnected network even though their phase fraction is low. This journal is