Abstract text (incl. figure legends and references)
Introduction

Pure Li₁Ti₂(PO₄)₃ (LTP) is an anode material with NASICON structure and a lithiation potential of 2.31 V.[1] This potential fits the electrochemical stability window of the promising isostructural solid state electrolyte Li_1+xAl_xTi_2-x(PO₄)₃ (LATP), which is obtained by trivalent substitution and application in an all-phosphate solid state battery has been demonstrated.[2] Comparison of different synthesis methods revealed an enhanced cycling performance of solvothermally prepared LTP with spindle-like particles of 2 – 5 µm size compared to the same material prepared by sol-gel based Pecchini method.[3]

Objectives

Scanning electron microscopy (SEM) suggests that spindle-like particles are formed by sub particles of about 300 nm size, but as only the surface is accessible by SEM the origin of the enhanced performance remains unclear. With focused ion beam (FIB) and (Scanning) Transmission Electron Microscopy the inner volume can be characterized to give a complete picture of the particles crystal- and micro-structure as well as its local chemical composition.

Materials and methods

The particles were synthesized by solvothermal reaction, consecutively vacuum dried and annealed at 800 °C.[3] These particles were then dispersed in ethanol and a droplet was put on a silicon wafer for FIB-SEM experiments. FIB tomography and TEM-lamella preparation were conducted with a Helios Nanolab 460F1, FEI, Netherlands.[4] TEM and STEM analysis was conducted in a Tecnai F20 and a Titan G2 Crewley.[5]

Results

SEM imaging shows the morphology, with sub particles forming a dumbbell like particle, and already provides hints for the presence of two secondary phases, on nanoparticles, the other a bulky phase with different surface morphology. The presence oof different phases is approved by chemical contrast in BSE-images of cross sections and STEM HAADF imaging. STEM-EDS gives an estimate of the chemical compositions which is completed by STEM-EELS for the detection of Lithium. HRTEM and HRSTEM could then identify the crystallographic structures of these secondary phases to be TiO₂ anatase for the nanoparticles and LiTiPO₅ (Pnma ICSD #153522) for the bulky secondary phase. FIB-tomography revealed that the majority of the TiO₂ nanoparticles are interconnected.

Conclusions

The complex microstructure of the spindle-like LTP particles can only be solved by a combination of FIB-tomography and STEM-analysis. The three-dimensional network of TiO₂-nanoparticles seem to improve the cycling behavior, as it may enhance the diffusion and can also contribute to capacity and is no dead material.[6]

[1] S. Yu et al., ACS Appl. Mater. Interfaces (2018) Vol. 10, No. 26 p. 22264-22277 https://doi.org/10.1021/acsami.8b05902

[2] H. Aono et al., Journal of The Electrochemical Society , (1990) 137, 4, p. 1023-1027 https://doi.org/10.1149/1.2086597

[3] S. Yu et al., ChemElectroChem (2016) Vol. 3, No. 7, p. 1157-1169 https://doi.org/10.1002/celc.201600125

[4] M. Kruth et al., Journal of large-scale research facilities JLSRF (2016) 2, A59 https://doi.org/10.17815/jlsrf-2-105

[5] A. Kovács et al., Journal of large-scale research facilities JLSRF (2016) 2, A43 https://doi.org/10.17815/jlsrf-2-68

[6] M. Madian et al., Batteries (2018) 4, 7, https://doi.org/10.3390/batteries4010007