Effects of fluorine doping in lithium iron pyrophosphate by studies of XRD, MAS NMR and galvanostat

Abstract

As a cathode material, lithium metal pyrophosphates, Li2MP2O7 (M = Mn, Co, Fe, V), have been subjected to intense recent research. Pyrophosphate of the kind have a two-dimensional lithium diffusion channel along the crystallographic b and c axis and extracting more than one lithium ion per formula unit is possible. Among them, Li2FeP2O7 (LFPO) shows superior ionic diffusivity, inferior electronic conductivity and structural stability due to strong P–O bond as polyanion. Its theoretical reversible capacity is 110 mAh g-1 per 1 e-. Anion doping into the polyanion site can be able to improve intrinsic electrochemical properties such as band gap and enhance lithium diffusion limitations. In this study, fluorine anion (F-) has been selected as a dopant in for oxygen sites. The fluorine doped LFPO has been synthesized by solid state reaction method. The structural study of the material has been investigated by X-ray diffraction and 7Li MAS NMR. And the electrochemical properties of samples have been carefully investigated through lithium extraction/inserting processes using galvanostatic charge/discharge and cyclic voltammetry (CV) measurements so as to investigate for Li ion transfer between the active electrode materials.