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Depth profiling of the Solid Electrolyte Interphase in Li-ion electrodes

Li-ion batteries (LIBs) represent a reliable, affordable, and safe energy storage technology for use in portable applications. However, current LIB active materials can store only limited energy since they rely on insertion storage based on solid-state host-guest interactions. Moreover, performances and durability of the cells are strongly influenced by the characteristics of the SEI (Solid Electrolyte Interphase) which is rapidly formed upon the electrodes, a process difficult to study and control. Recently, research efforts have been thus devoted both in devising new active materials and in more accurate investigations of the SEI evolution mechanisms.

In particular, alternative charge/discharge mechanisms have been explored with the aim of reaching higher energy densities using raw materials with lower costs and environmental impact. In this context, zinc iron oxide (ZnFe2O4, ZFO), encapsulated by a carbonaceous matrix (ZFO-C) has been recently developed as an innovative anode material. This system has been found to be able to exchange Li+ and e− both by conversion and alloying processes. As a consequence Fe, LiZn, Li2O are formed upon lithiation, which are finely dispersed into a carbonaceous matrix. While the lithiation kinetics have already been probed by electrochemical impedance spectroscopy (EIS) and x-ray diffraction (XRD) analysis, very little is known about the evolution of passivation layer properties on ZFO-C.

In a recent experiment, performed within a European (FP7 project SIRBATT) collaboration effort, we have studied the evolution of the SEI in this innovative anode material at selected charging steps by exploiting the surface sensitivity of the soft x-ray absorption spectroscopy (XAS). This technique requires synchrotron radiation and was never used before for such a purpose, although it appears to be very suitable for a detailed depth profiling of the SEI of advanced electrodes. Accurate XAS experiments on ZFO-C anodes, at different stages of SEI development, were performed in the 50-1000 eV photon energy range at the BEAR beamline of the Elettra synchrotron laboratory using both total electron (TEY) and total fluorescence (TFY) yield. Those detection modes show effective probing depths around 2-10 nm and 70-200 nm, respectively, and are highly complementary for a complete study of the SEI growth. Ex-situ TEY and TFY x-ray absorption experiments have been conceived to study the modification of the signals related to the various atomic species in ZFO electrodes selected at different states of charge. XAS measurements have been preceded and corroborated by a complete characterization including electrochemical impedance spectroscopy (EIS), for crosschecking the SEI evolution with the polarization of the electrodes. We have found that the evolution of the SEI takes place already during the first steps of the charging process and its thickness reach about 40 nm at a voltage of 0.79 V (417 mAh/g) starting from the de-lithiated electrode (2.53 V, 0 mAh/g) and then it is almost stable up to 20 working cycles. By careful analysis of XAS signals related to the ZFO particles and to the binder, we have found that the SEI grows preferentially around the ZFO nanoparticles.  These findings are extremely useful for the development of new techniques and materials aimed to improve stability and storage capabilities of Li-ion batteries.

Figure 1. Top: SEI thickness as a function of capacity during first Li uptake and after 20 full charge/discharge cycles, as estimated by XAS intensity decay. The estimated SEI thickness as detected by atoms included in the ZFO nanoparticles (Zn, Fe) grows up to about 40 nm, while that related to Na atoms (CMC binder) is limited to about 15 nm (lines are guide for the eye). Bottom: Pictorial view of the SEI formation. In an initial stage (left) a thin SEI is formed by electrolyte decomposition, while the electrolyte begins to percolate inside the porous electrode structure. In an intermediate phase during the Li uptake (center), the electrolyte is fully percolating inside the structure and the SEI grows around the active nanoparticles. At a later stage (right-hand) a thicker SEI is formed preferentially upon the ZFO nanoparticles.

 

This research was conducted by the following research team:

  • A. Di Cicco, R. Gunnella, F. Nobili, M. Pasqualini, R. Tossici, School of Science and Technology University of Camerino, Camerino, Italy
  • F. Mueller, S. L. Koch, S. Passerini, Helmholtz Institute Ulm, Ulm, and Karlsruhe Institute of Technology, Karlsruhe, Germany.
  • A. Witkowska, Dept. of Solid State Physics, Gdansk University of Technology, Poland
  • A. Giglia, CNR, Istituto Officina Materiali, Trieste, Italy


Contact person:

Andrea Di Cicco, E-mail: andrea.dicicco@unicam.it

 

Reference

A. Di Cicco, A. Giglia, R. Gunnella, S. L. Koch, F. Mueller, F. Nobili, M. Pasqualini, S. Passerini, R. Tossici, A. Witkowska, “SEI growth and depth profiling on ZFO electrodes by soft x-ray absorption spectroscopy”, Advanced Energy Materials 1500642 (2015),  doi:10.1002/aenm.201500642

 

Last Updated on Friday, 24 July 2015 11:11