Xpeem Reveals the Local (electro-) Chemical Surface Reactions of the Ncm Cathode and Their Impact on the Lto Counter Anode
Authors: Leanza Daniela , A. F. Vaz Carlos , Novák Petr , El Kazzi Mario 
Published: February 2020

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 (poor lateral resolution). Thus, in this contribution we use X-Ray Photoelectron Emission Spectroscopy (XPEEM) as a surface analytical technique capable of providing localized information on single particles of composite anode and cathode materials, while preserving their working environment (as in commercial-like 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 finally provide the missing piece of the exact mechanism of the electrolyte-electrode interactions. The surfaces of two types of electrodes are investigated and will be presented, Li(Mn,Ni,Co)O2-Li2MnO3 (NCM) and Li4Ti5O12 (LTO), as promising industrially relevant positive and negative electrodes, respectively, for Li-ion batteries. A systematic study of the potential dependency of the surface evolution was performed during the early stage and after long cycling. The contrast images in Figure 1a,d attest the good spatial resolution of XPEEM and its capability to localize the different particles present on the surface. The local XAS spectra (Figure 1b) at the transition metals (TMs)-L edges acquired on NCM particles allow us to monitor its oxidation states and the surface structure modification. At the same time, the C-K edge provides direct information on the chemical changes and the electrolyte byproducts decomposition.[1] We find that, despite cycling the NCM to a highly oxidative potential of 5.1 V vs. Li+/Li, there is no formation of an oxidized electrolyte byproduct layer on the surface of the cathode particles (C K-edges Figure 1.b, c); instead, we show that a homogeneous organic-inorganic surface layer builds-up across the particles of the LTO counter anode (C K-edges, Figure 1.e f). In addition, we observe that such surface layer incorporates, already from the first charge, micrometer-sized agglomerates of TMs explained as due to the instability of a reduced layer of Mn, Co and Ni at the NMC-electrolyte interface that is mainly formed during the cathode delithiation (Mn L-edge, Figure 1.b). These results demonstrate that two simultaneous reactions take place at the NCM-electrolyte interface subject to a high working potential, namely, surface degradation of the NCM active materials and decomposition of the electrolyte which affect directly the anode surface and the overall cell performance through a cross-talk process.
[1] D. Leanza, C.A.F. Vaz, I. Czekaj, P. Novák, M. El Kazzi, Journal of Materials Chemistry A 6 (8), 3534-3542 (2018)

Figure 1