Revealing the Dual Surface Reactions on a HE-NCM Li-Ion Battery Cathode and Their Impact on the Surface Chemistry of the Counter Electrode

The understanding of surface reactions at the electrode–electrolyte interfaces has been a longstanding challenge in Li-ion batteries. X-ray photoemission electron microscopy is used to throw light on the disputed aspects of the surface reactivity of high-energy Li-rich Li1+x(NiaCobMn1–a–b)1–xO2 (HE-NCM) cycled in an aprotic electrolyte against Li4Ti5O12 (LTO). Despite the highly oxidative potential of 5.1 V vs Li+/Li, there is no formation of a layer of oxidized electrolyte byproducts on any of the cathode particles; instead, a homogeneous organic–inorganic layer builds up across the particles of the LTO anode due to the electrolyte and poly(vinylidene fluoride) binder decomposition on HE-NCM. In addition, such a layer incorporates, already from the first charge, micrometer-sized agglomerates of transition metals (TMs). The presence of TMs on the anode is explained by the instability of the reduced Mn, Co, and Ni formed at the surface of HE-NCM mainly during delithiation. The reduced TMs are unstable and prone to be transported to the LTO, where they get further reduced to metallic-like clusters. These results demonstrate that a dual reaction takes place at the HE-NCM–electrolyte interface if subject to high potential, namely, degradation of the surface structure and decomposition of the electrolyte, affecting directly the anode surface through the migration–diffusion processes.