(Invited) Nanoscale (XPEEM) Spectroscopy Unveils the Complex Local (Electro-) Chemical Surface Reactions of Cycled Electrodes in Li-Ion Batteries

Abstract

Understanding the electrode-electrolyte interface evolution during cycling as well as the surface structural change of the active materials is crucial to improve the safety and the life-cycle of Li-ion batteries. Despite the large effort directed at elucidating the nature and origin of the surface reactivity of the electrodes in liquid organic electrolytes, there is still no complete knowledge of the various mechanisms that take place at such interfaces. The lack of agreement in the reported findings is caused mainly by the surface complexity of the commercial electrodes (multiple particles, high roughness and porosity) and by the intrinsic limitations of the commonly used surface characterization techniques, (mainly the poor lateral resolution). Thus, in this contribution the recent development on X-Ray Photoelectron Emission Spectroscopy (XPEEM) will be presented, as a surface analytical technique capable of providing local information on single particles of anode and cathode materials, while maintaining the complex formulation of composite electrodes. The unique combination of the nanoscale lateral resolution and the spectroscopic capability, confined to within the depth analysis range of 3-4 nm, enable us to provide the missing piece of the exact mechanism of the electrolyte-electrode interactions. Two commercially-relevant electrodes for Li-ion battery will be presented during the talk, namely Li4Ti5O12 (LTO) and the high-voltage cathode lithium rich Li1+x (NiaCobMn1-a-b)1-xO2 (Li-rich NCM) after cycling in the conventional liquid carbonate electrolytes. In the first system, the controversial surface reactivity of LTO when is cycled in carbonate-based electrolyte is resolved. Despite the theoretical prediction of a stable electrochemical interface, the electrolyte reduction is found to occurs solely on LTO particles during lithiation. With the support of density functional theory (DFT) calculations, we show that this behavior is caused by the solvents adsorbed on the LTO outer planes driven by the Li-ion insertion. The DFT calculation indicates that Li-ion insertion leads to a shift of the LUMO of the adsorbed solvents to energies below the Fermi level position of lithiated Li7Ti5O12 and thus to chemical instability.[1]In the second system, at the Li-rich NCM interface, two major parasitic reactions were identified at high potential. The first reaction is related to the electrolyte oxidation, whereas the second is associated with the Li-rich NCM surface degradation.[2] Those parasitic reactions are caused mainly by the oxidation of oxygen leading to the chemical oxidation of the electrolyte solvents and to the formation of a surface layer containing reduced transition metals (TMs).[3] The presence of electrolyte by-products and TMs "cross-talk" is also demonstrated between the cathode and the anode, creating on the latter a layer rich of micrometer-sized agglomerates of TMs. The impact of these parasitic reactions on the electrodes electrochemical cycling performance will be discussed. Finally, as a further development of the technique, operando XPEEM will be presented and discussed on all-solid-state batteries for the working electrode LiCoO2 mixed with (Li2S)3-P2S5 (LPS), used as a solid electrolyte. [1] D. Leanza, C. A. F. Vaz, I. Czekaj, P. Novák, M. El Kazzi, Journal of Material Chemistry A, 2018, 6, 3534–3542. [2] D. Leanza, C. A. F. Vaz, G. Melinte, X. Mu, P. Novák, M. El Kazzi, ACS Appl. Mater. Interfaces, 2019, 11, 6054−6065. [3] D. Leanza, M. Mirolo, C. A. F. Vaz, P. Novák, M. El Kazzi, Batteries & Supercaps, 2019, 2, 1-12.