Investigation of the Electronic and Local Structural Changes during Li Uptake and Release of Nanocrystalline NiFe2O4 By X-Ray Absorption Spectroscopy

Abstract

As the most promising power sources for smart phones, laptops, electric vehicles (EVs) and hybrid electric vehicles (HEVs), lithium ion batteries have attracted much attention in past decades. In order to meet the ever-increasing demand for high energy density, high capacity and long service life, various transition metal oxides and compounds (TMOs) have been investigated because of their much higher theoretical capacity based on the conversion reactions [1-3]. During Li uptake, TMOs are firstly reduced to extremely small metallic nanoparticles dispersed in a Li2O matrix. And during subsequent charging, these metallic particles are oxidized. The reaction mechanism can be described using the following general equation [1]: TM x O y + 2yLi+ +2ye- → xTM + yLi2O (1) Among these TMOs, iron based compounds are particularly interesting candidates to investigate as they are less expensive and more environmental friendly than cobalt contained oxides. As a member of iron based compounds, NiFe2O4 comes into notice since it contains electrochemically active nickel and displayed a high theoretical capacity of 914 mAh g-1. However, these conversion reaction electrodes showed a fast capacity fading in few cycles because of the formation of an insulating polymeric layer around the particles and poor integrity caused by the drastic volume changes upon cycling [4-5]. Although the conversion reaction 1 of simple monoxides (CoO, CuO, FeO or NiO)[1] is thermodynamically favorable and has been widely investigated by ex situ/in situ X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS) and Mössbauer spectroscopy during discharge and charge. Lots of detailed information for the reaction is still not well understood yet due to the intriguing nature of amorphous Li2O and ultrafine metal particles and limitations for the above techniques. A better understanding of the reactions occurring during Li uptake and release is crucially important to search for new high performance electrode materials. In this work, nanocrystalline NiFe2O4 particles were successfully synthesized and used as active electrode material for lithium ion battery. X-ray absorption spectroscopy (XAS) was applied to investigate the local structural changes and electrochemical reactions of NiFe2O4 in the cell during 1st discharge and charge process. As lithium inserted into the structure, Fe3+ ions are firstly reduced to Fe2+, but Ni2+ ions are unaffected which can be seen in fig.1. The matrix spinel structure collapses and transform to a rock salt monoxide phase with further lithiation. Along with the conversion reaction, the formation of SEI on the electrode consumed a large amount of lithium ions. The rock salt monoxides are reduced to metallic products at the end of discharging with a small particle size and highly disorder. The refined EXAFS results show that these metallic Fe/Ni nanoparticles are not well refined with a nearest number of neighbors of 6 and 8 for Fe and Ni respectively. All these ultrafine metal particles are reoxidized to Fe2O3and NiO phase instead of pristine spinel structure during the following charging. Reference [1] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.M. Tarascon, Nature 407 (2000) 496-499. [2] R. Alcántara, M. Jaraba, P. Lavela, J.L. Tirado, J.C. Jumas, J. Olivier-Fourcade, Electrochemistry Communications 5 (2003) 16-21. [3] J. Cabana, L. Monconduit, D. Larcher, M.R. Palacín, Advanced Materials 22 (2010) E170-E192. [4] X. Guo, X. Lu, X. Fang, Y. Mao, Z. Wang, L. Chen, X. Xu, H. Yang, Y. Liu, Electrochemistry Communications 12 (2010) 847-850. [5] J.-M. Tarascon, S. Grugeon, M. Morcrette, S. Laruelle, P. Rozier, P. Poizot, Comptes Rendus Chimie 8 (2005) 9-15. Figure 1