Investigation of the cathode material Li9V3(P2O7)3( PO 4)2 for Li‐batteries using Cs‐corrected HRTEM

Abstract

New cathode materials for Li‐ion batteries are investigated assiduously to achieve higher performance in terms of energy density life time and safety. In pursuit, a polyanion‐based lithium vanadium monodiphosphate, Li 9 V 3 (P 2 O 7 ) 3 (PO 4 ) 2 [LVPP] was identified. This layered material poses distinct advantages like higher energy density, thermal stability over the more commonly used metal oxides, owing to the strong binding of the phosphate anions. Theoretically the extraction of maximum 6 Li ions per unit cell is possible, thus giving out a theoretical capacity of 173mAhg ‐1 through complete oxidation of vanadium from its initial +3 to final +5 state. A facile synthesis method of LVPP has been designed. To comprehend the structure and its functionality, transmission electron microscopy (TEM) with image Cs‐corrector was used (FEI Titan 80‐300kV), allowing atomic resolution even at lower accelerating voltages [2] (in this case at 80kV), where lower knock‐on damage is expected. To preserve as good as possible the original quality of the synthesized powder material, the TEM specimen was prepared without additional grinding using three methods: simply spread of powder particles on the carbon foil, sonicate powder in alcohol and fishing particles on the carbon foil and spread of particles, which were crashed after cooling in liquid nitrogen, on the carbon foil. The crystallographic structure of LVPP corresponds to the trigonal symmetry (space group P‐3c1) with a = b = 90°, g = 120°, a = b = 9.724 Å , c = 13.596 Å [3]. VO 6 octahedra and PO 4 tetrahedra, connected to the Li atoms, are organized in two layers generating a 2D ionic conductivity. Fig.1d) shows the simulated structure in [0001]‐ and [‐2021]‐projections where V atoms are red, P atoms blue, O atoms light rosa and Li atoms yellow. HRTEM images of thin particles edges at 80 kV and 300kV are shown in Fig. 1 a‐c). To get interpretable TEM micrographs usually the crystalline specimen has to be aligned during imaging in a particular pre‐selected crystal orientation. However LVPP is very sensitive to the electron beam with the result that practically the crystal could not be oriented. This behaviour was found independent of the accelerating voltage of 80kV or 300kV. Fortunately, some particles had by chance a favorable crystal orientations and qualitatively interpretable TEM images were obtained (see Fig. 1). We confirmed the expected crystallographic structure for the synthesized particles by corresponding image simulations using JEMS. Moreover we found no significant volume change of the structure during electron‐beam driven delithiation.