Insights into the Cathode-Electrolyte Interphases of High-Energy-Density Cathodes in Lithium-Ion Batteries

Abstract

We present a comprehensive study of cycled high-Ni (LiNi_1-xM_xO₂, M = metals), Li-rich (Li_1+xMn_yM_1-x-yO₂), and high-voltage spinel (LiMn_1.5Ni_0.5O₄) electrodes with time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy in conjunction with electrochemical techniques to better understand their evolving cathode-electrolyte interphase structure during cycling. TOF-SIMS provides fragment-specific information regarding the surface film content for each of the electrodes. High-Ni cathodes show thick surface films initially containing Li₂CO₃, later developing oxidized organic carbonates throughout cycling. Li-rich electrode surface films develop strong characteristics during their first activation cycles, where released O₂ oxidizes organic carbonates to form polymeric carbons and decomposes LiPF₆. High-voltage spinel electrodes operate outside the standard electrolyte stability window, generating reactive oxidized electrolyte species that further decompose LiPF₆. The distribution and concentration of these different chemical fragments measured by TOF-SIMS are finally summarized by color-coded high-resolution images of cycled high-Ni, Li-rich, and high-voltage spinel electrodes.

Keywords: electrochemistry; high-energy-density cathodes; interphases; lithium-ion batteries; surface analysis.