The interphase chemistry in Li-ion batteries studied by electronic and ionic spectroscopy 

J. Światowska

Laboratoire de Physico–Chimie des Surfaces, CNRS (UMR 7045), École Nationale Supérieure de Chimie de Paris (Chimie ParisTech), Paris, France

The interphase chemistry in Li-ion batteries (LIBs) is mainly related to reactions at the electrode/electrolyte interfaces where passive films can be formed on the negative or positive electrode. The formation of the passive film on the negative electrode, called Solid Electrolyte Interphase (SEI) layer¹, is due to the reductive decomposition of the electrolyte at a potential around 0.7 V vs. Li/Li⁺, which is higher than the potential of electrode lithiation (intercalation, alloying or conversion). The SEI layer can have different physicochemical properties (i.e. ionic and electronic conductivity, thickness, mechanical properties etc.) The properties of SEI layer affect the LIBs cyclability, life time, power and rate capability, and even their safety. Therefore, the understanding of the formation of the SEI layer and the possibility of tuning its quality on the surface of the negative electrodes are essential for optimizing the LIB systems.

In our laboratory two principal surface sensitive techniques particularly adapted for analysis of surface reactions like formation of passive films are used: X–ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS), both of them directly connected to glove-boxes. XPS is used to investigate the surface chemical composition of electrode materials by analysis of valence band region and electronic core levels. As this technique provides information on the first few nanometers at the electrode surface, depth profiling analysis is also necessary for the characterization of the passive films and cycled bulk materials. The depth profile can be either performed by means of XPS or ToF–SIMS using Ar or Cs ions, respectively, for sputtering. ToF-SIMS is a highly sensitive surface analytical technique where a pulsed primary ion beam (e.g. Bi⁺) is used to extract secondary ions that are analyzed by time–of–flight spectrometry. Interlaced with a sputtering ion beam (e.g. Cs⁺), compositional depth profiles with excellent depth resolution (monolayer) and high sensitivity (ppb) can be readily obtained. ToF–SIMS analysis can be applied to cycled electrode materials providing information about the passive layer and the depth and lateral distribution of species in the bulk electrode materials.

Our approach is to use model samples (i.e. thin film electrodes grown directly on current collector) with a good surface finishing presenting a minimum roughness adapted to the analysis by surface sensitive techniques.

Few examples showing the negative electrode/electrolyte interphase reactions with application of XPS and ToF-SIMS techniques will be given in this presentation:

already after the first cycle of charge/discharge due to formation of the SEI layer by reductive decomposition of electrolyte. The formation of the SEI layer on the negative electrode material is irreversible and its thickness can increase when increasing the number of charge/discharge cycles. Some information about the differences in SEI layer composition formed on the Si electrodes in different electrolytes will be given.

ToF-SIMS depth profiles data show variation of film thickness with lithiation/delithiation process. ToF-SIMS evidences also a trapping of lithium and products of electrolyte decomposition in the bulk of electrode material.

 References

¹ E. Peled, Journal of The Electrochemical Society 126 (1979) 2047-2051.