Tin-based hydrocarbon nano-particulate composites are referred to advanced graphite anodes, and have been shown to be highly promising materials to enhance the performance of lithium ion batteries by optimizing the specific capacity. The key point of this article is the development of a polymeric solid electrolyte interphase (SEI) during the initial electrical cycles of the battery, and the SEI’s chemical composition is necessary for additional developments with regard to long-term and cycle stability.
The Thermo Fisher Scientific™ K-Alpha™ XPS instrument (Figure 1) was used to characterize nano-particular tin-based electrode composites for lithium-ion batteries (Figure 2) in the pure status and following polymeric SEI formation. Post mortem X-ray photoelectron spectroscopy is one of the fundamental surface analytical techniques to obtain the preferred chemical data in a non-destructive manner.
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Figure 1. Thermo Fisher Scientific™ K-Alpha™ XPS instrument
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Figure 2. Lithium batteries
Experiment
In order to synthesize electrode, porous SnO2/CXHY nano-particular films were directly placed onto preheated nickel current collectors with the aid of the Karlsruhe Microwave Plasma Process (KMPP) using water-free Sn(C4H9)4 as a precursor in pure Ar carrier gas.
For post mortem XPS characterization, the electrodes were discharged from 2.8 to 0.8 V (predicted reduction of electrolyte and SnO) and from 2.8 V down to 0.25 V (predicted alloying of tin with lithium) using vinylene carbonate (VC) and LP30 electrolyte as additive.
To make XPS measurements, newly prepared samples were placed within a glove-box that was directly fixed to a load lock of the Thermo Scientific K-Alpha instrument. The whole range of samples was examined with the micro-focused, monochromated Al Kα X-ray source at 400 µm spot size. During examination, the charge compensation system was used to stop any localized accumulation of charge. Next, the Thermo Scientific Avantage software was used to perform data acquisition and processing, and the spectra acquired were subsequently fitted with one or more Voigt profiles (binding energy uncertainty: +/- 0.2 eV). All of the acquired spectra were referenced to the C 1s peak of hydrocarbons at a binding energy of 285.0 eV.
Results
Figure 3 shows the O 1s, C 1s, and Li 1s spectra of the pure electrode surface and surfaces of two cells cycled down to 0.8 V and 0.25 V, respectively.
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Figure 3. C 1s, O 1s, and Li 1s XP spectra of a pristine electrode surface and electrodes surfaces cycled to 0.8 V and 0.25 V, respectively.
The enhanced electrochemical performance of VC comprising cells is chiefly due to the development of polymeric surface species arising from VC, and this was validated by the components at a very high binding energy of O 1s = 534.5 eV and C 1s = 291.0 eV. Apart from the VC-based polymer, the SEI, formed also contains additional decomposition products of the electrolyte subsequent to discharging from 2.8 to 0.25 V, shown by the peaks at 286.6 eV, 287.6 eV and 289.0 eV, and these can be linked to alkoxy-, ethereal-, carbonyl-, and alkylcarbonate species. These findings are confirmed by the relative O 1s peaks at 531.6 eV, 532.1 eV and 533.0 eV. These compounds occur from the direct decomposition of the carbonates present in the LP30 electrolyte. The appearance of peaks proves the extra decomposition of the conducting salt (LiPF6). These peaks are linked to lithium fluoride and a number of fluorinated lithium phosphate species (F 1s at 685.1 eV, and F 1s at 687.3 eV, P 2p3/2 at 134-137 eV).
In Figure 3, the Li 1s spectra of both cycled electrode surfaces reveal relatively wide peaks, which were not additionally deconvoluted because of the low sensitivity for lithium and minimal binding energy shifts between varied lithium species. However, the tailing at lower binding energies suggests the presence of Li2O and is also established by the corresponding O 1s part at 528.2 eV.
Conclusion
It has been shown that the XPS is a powerful and indispensable tool, which can be effectively used to characterize the SEI of Lithium-ion batteries in a simple and non-destructive way. In this particular study, the XPS technique could confirm the formation of a primarily polymer containing SEI based on the VC additive and additional compounds originating from the decomposition of the conducting salt as well as the electrolyte.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – X-Ray Photoelectron Spectroscopy (XPS).
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