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

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.