How Bulk Sensitive is Hard X-ray Photoelectron Spectroscopy: Accounting for the Cathode–Electrolyte Interface when Addressing Oxygen Redox

Abstract

Sensitivity to the "bulk" oxygen core orbital makes hard X-ray photoelectron spectroscopy (HAXPES) an appealing technique for studying oxygen redox candidates. Various studies have reported an additional O 1s peak (530−531 eV) at high voltages, which has been considered a direct signature of the bulk oxygen redox process. Here, we find the emergence of a 530.4 eV O 1s HAXPES peak for three model cathodesLi 2 MnO 3 , Li-rich NMC, and NMC 442that shows no clear link to oxygen redox. Instead, the 530.4 eV peak for these three systems is attributed to transition metal reduction and electrolyte decomposition in the near-surface region. Claims of oxygen redox relying on photoelectron spectroscopy must explicitly account for the surface sensitivity of this technique and the extent of the cathode degradation layer.

Effects of Film Formation on the Electrodeposition of Lithium

“…HAXPES has gained recent popularity in the analysis of battery materials because it offers surface analysis at deeper locations than that by XPS. [38][39][40][41] XPS with Ar sputtering can be used to perform depth analysis, but redeposition and decomposition of the sample will occur and must be taken into consideration. The Li1s and C1s XPS and HAXPES spectra of the substrate obtained in the electrolyte with 100 ppm water are given in Figure 9.…”

“…The surface film of densely packed lithium obtained in the electrolyte with 100 ppm water was evaluated using XPS and hard X‐ray photoelectron spectroscopy (HAXPES). HAXPES has gained recent popularity in the analysis of battery materials because it offers surface analysis at deeper locations than that by XPS [38–41] . XPS with Ar sputtering can be used to perform depth analysis, but redeposition and decomposition of the sample will occur and must be taken into consideration.…”

Deciphering the Oxygen Absorption Pre‐edge: A Caveat on its Application for Probing Oxygen Redox Reactions in Batteries

“…All the TEY and TFY data of total 55 samples are provided in Figures . Because the surface signals may not be intrinsic to the electrode material property and could vary upon electrochemical cycling, [ 32 ] our analysis in this work focuses on the relatively bulk‐sensitive TFY spectra, especially for samples displaying the contrasts between surface and bulk signals, as also mentioned in .…”

Phosphoric acid and thermal treatments reveal the peculiar role of surface oxygen anions in lithium and manganese-rich layered oxides

“…$4.5 V Li + /Li ). [8][9][10] Among these compounds, Li-and Mn-rich layered oxides (LMLOs) are at the forefront of this area achieving high discharge capacities above 250 mA h g À1 . 11,12 However, the oxygen redox activity always causes changes in the crystallographic structure, surface inhomogeneity and release of lattice oxygen in LMLOs, thereby resulting in structural instability and severe voltage fading.…”