Surface effects on electrochemical properties of nano-sized LiFePO4

International audienceLiFePO4 has won the challenge to be the active element for the positive electrode of Li-ion batteries for electromobility. In an attempt to optimize the electrochemical performance, efforts have been made to reduce the size of the particles, so that the electrons and Li+ ions have a reduced path to travel inside the material. However, when the size decreases below 100 nm, surface effects become increasingly important, and can eventually dominate the physical and the chemical properties. The purpose of this work is to review them and their implications, involving sensitivity to exposure to humidity, and separate between surface disorder effects, impurities and intrinsic properties. This goal is achieved by the combination of different techniques to characterize the particles, including X-ray diffraction, electron microscope imaging, optical spectroscopy, and magnetism. 1. Introduction LiFePO4 has been subjected to extensive studies as a positive electrode material for a new generation of Li-ion batteries since the pioneering work of Padhi et al.1 Carbon coating has overcome the drawback of highly resistive LiFePO4 powder.2-5 In addition, the material purity is now under control owing to reduction of iron by hydrogen gas present in the course of the synthesis, either because the samples are prepared by hydrothermal synthesis,6 or because the polymer additive frees hydrogen gas in different chemical routes.7 Attention is now mainly focused on attempts to increase the rate capability (for use in hybrid electric vehicles for instance). It is commonly recognized that this goal could be achieved by decreasing as much as possible the size of the particles to improve the effective surface that is active for electrochemical reactions. In addition, smaller size means reduced path for the electrons and the Li+ ions inside the particles. Since the electronic and ionic conductivity are small,8,9 this reduction is expected to benefit the performance, especially at high C-rates. For these reasons, extensive effort has been devoted to reduce the particle size in recent years.10-12 The experimental results, however, are not as simple as one might have expected, because the reduction in size implies that surface effects become more important, and the surface layer does not necessarily have the same properties as the bulk, with important impact on the electrochemical properties. The bulk properties (i.e. physical and chemical properties big enough so that surfac