Interphase Chemistry and Li-Ion Transport in Negative Electrode Materials Studied by XPS and ToF-SIMS
Two principal surface-sensitive techniques are particularly suitable for analyzing surface reactions at electrodes: X–ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). As XPS provides information on chemical composition on the first few nanometers at the electrode surface, depth profiling analysis is also necessary for the characterization of thicker surface layers and bulk materials. Depth profiling can be either performed by 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+), elemental depth profiles with excellent depth resolution (monolayer) and high sensitivity (ppb) can be readily obtained. ToF-SIMS can also serve as a new and more direct method than the commonly used electrochemical methods (i.e. electrochemical impedance spectroscopy, cyclic voltammetry or galvanostatic methods) for precise measurement of ion diffusion in electrode materials.
Our approach is to use model thin-film electrodes (i.e. from few tens to few hundreds of nm directly grown on a current collector) with a good surface finishing and having a roughness suitable for analysis by surface sensitive techniques. The application of model thin-film electrode having enlarged surface-to-volume ratio provides clearer and more comprehensible insight into electrode/electrolyte interface reactions without complications from current percolators or binding agents that are used in bulk composite electrode materials. This allows for studies of intrinsic electrochemical and interfacial processes occurring on the electrode surfaces.
In the present work, XPS and ToF-SIMS have been used to investigate the chemical and volume modifications of thin-film negative-electrode materials. As an example, two types of high capacity negative-electrode materials will be presented:
- a conversion-type electrode consisting of iron oxide (with a theoretical capacity of 1007 mAh/g), and
- an alloying-type electrode consisting of silicon (with a theoretical capacity of 3579 mAh/g).
Both negative-electrode materials are interesting and important candidates for application in LiBs due to their abundance and environmental friendliness.
For the first time, ToF-SIMS depth profiling have been applied to measure an apparent diffusion coefficient of lithium ions (DLi+) into these electrodes. The DLi+was calculated from the finite integration of Fick’s second law for one-dimensional diffusion.
Regardless of the type of electrode material, XPS analysis shows strong intensity attenuation and binding energy positive shift of the principal core level peak corresponding to the main electrode component (Si2p in the case of Si-electrode and Fe2p in the case of Fe2O3-one), confirming the formation of the SEI layers on the surfaces after cycling in PC-LiClO4. The chemical composition of the SEI formed on the two different electrodes is essentially identical, with a principal Li2CO3 constituent and a minor quantity of alkyl carbonates (ROCO2Li) as evidenced by the C1s, O1s and Li1s core level peaks [2,3]. The SEI layer chemistry principally depends on the electrolyte composition .
Both, XPS and ToF-SIMS, evidence a dynamic increase/decrease of the SEI layer thickness upon lithiation/delithiation, superimposed to a continuous increase upon cycling. The data reveal the formation of duplex-like SEI layer structure, with the organic carbonate species in the outer region and inorganic salt decomposition products in the inner part of the SEI layer. The ToF-SIMS depth profiles also show irreversible Li trapping in the bulk electrodes and volume variations with the degree of lithiation and the number of cycles [2,4]. These modifications can be detrimental to electrode cycling and capacity retention.
A low DLi+ in the order of 10-15 cm2/s calculated from ToF-SIMS depth profiles for these two types of electrodes, indicates that the rate of lithiation is slower than for intercalation-type materials and can have an important influence on their electrochemical performance.
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