Understanding the active formation of a cathode–electrolyte interphase (CEI) layer with energy level band bending for lithium-ion batteries

Abstract

Cathode–electrolyte interphase (CEI) formation between the cathode and the electrolyte is a critical factor that determines the stability of lithium-ion batteries (LiBs). The CEI layer consists of various by-products (e.g., LiF, Li₂CO₃, ROLi, and ROCO₂Li (R: alkyl group)) decomposed from redox reactions between the cathode and the electrolyte, which can lead to dramatic capacity fading and stability issues. Herein, we empirically identify the energy level band bending of a Ni-rich NMC cathode (i.e., Li(Ni_0.5Mn_0.3Co_0.2)O₂) with the visual evidence of Li⁺ transfer from the electrode to the CEI layer (adsorbate). Negatively charged elements tend to be present at the close surface of the cathode, while the positively charged Li⁺ migrates from the cathode to the CEI layer. Hence, a downward band bending could be depicted based on the work function and the energy level difference between the Fermi level (E_F) and the valence band maximum (E_VBM). Energy level alignment itself is likely to be the key process that leads to the active formation of unstable CEI layers on charge–discharge.