Elucidation of the Complex (De-)Lithiation Reactions of SnO2 in All Solid-State Battery Using Operando X-Ray Photoelectron Spectroscopy
Authors: Mirolo Marta , Wu Xiaohan , A. F. Vaz Carlos , Novák Petr , El Kazzi Mario 
Published: February 2020

For the successful employment of Li-ion batteries at large scale, e.g. for electrical vehicles or stationary energy storage, a crucial point to be solved is the development of safer, non-flammable batteries. For that purpose, the currently used carbonate-based liquid electrolyte has to be substituted with a non-flammable material such as solid-state electrolytes. In particular, (Li2S)3-P2S5 (LPS) is among the promising solid electrolyte (SE) materials showing a good ion conductivity at room temperature (~ 0.4 mS/cm)1.
Another improvement that is also needed in the near future concerns the energy density of the battery. In order to increase this parameter, anode conversion-alloy materials, such as SnO2, are a serious choice, with a specific capacity of ~ 1500 mAh/g. Despite the theoretical predictions of the different SnO2 conversion and alloy reactions during (de-)lithiation, the experimental identification of those electrochemical reactions and the formation of intermediate species (e.g. LiaSnOb, LixSn and Li2O) is not fully understood2, mainly due to their possible relaxation and conversion to other byproducts during disassembling of the cycled cell in post mortem analysis3. Li2O, in particular, converts into the more stable Li2CO3 phase in presence of traces of CO2.
In this contribution, we will show how we recently developed operando X-ray photoelectron spectroscopy (o-XPS) method, offering an unprecedented manner of monitoring in real time the evolution of the electrolyte-electrode interface and the (de-)lithiation processes of the active materials during solid state Li-ion battery operation4. In this study, o-XPS is employed to investigate the (de-)lithiation of the SnO2 particles in a working electrode composed of SnO2 nanoparticles, LPS SE and Super P as conductive carbon cycled versus InLix counter electrode (Figure 1a). The analysis of the Sn 3d (Figure 1b), Sn 4d, O 1s (Figure 1c) and Li 1s spectra reveals the progressive conversion of the SnO2 particles to form Sn0 and the simultaneous formation of the LixSn alloy. When the potential is below 0.4 V (vs. Li+/Li), the Li2O phase is formed and grows until the full lithiation at 0.01 V (vs. Li+/Li), where the only species observed are Li2O and LixSn. By avoiding the conversion of Li2O to Li2CO3, we are also able to follow the de-lithiation process, confirming the reversibility of the conversion-alloy reactions where LixSn is converted to SnOx at 2.3 V (vs. Li+/Li). The S 2p and P 2p core levels reveal the presence of reduced Li2S byproduct species below 1.6 V (vs. Li+/Li), in accordance with previous studies1.
The knowledge gained with o-XPS on the (de-)lithiation processes of SnO2 and the stability of the SnO2-LPS interface is crucial to understand the mechanism leading to capacity loss during cycling and help to design a better cycling protocols for the SnO2 electrodes in solid-state batteries.

References:

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Cheng, A. Nie, L.-Y. Gan, Q. Zhang, U. Schwingenschlögi, J. Mater. Chem. A, 2015, 3, 19483

Ferraresi, C. Villevieille, I. Czekaj, M. Horisberger, P. Novák, M. El Kazzi, ACS Appl. Mater. Interfaces, 2018, 10, 8712-8720

X. Wu, C. Villevieille, P. Novák, M. El Kazzi, Phys.
Chem. Chem. Phys., 2018, 20, 11123

Figure 1