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

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-Li2MnO3, 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

Surface Chemistry Dependence on Aluminum Doping in Ni-rich LiNi 0.8 Co 0.2− y Al y O 2 Cathodes

Abstract: Aluminum is a common dopant across oxide cathodes for improving the bulk and cathode-electrolyte interface (CEI) stability. Aluminum in the bulk is known to enhance structural and thermal stability, yet the exact influence of aluminum at the CEI remains unclear. To address this, we utilized a combination of X-ray photoelectron and absorption spectroscopy to identify aluminum surface environments and extent of transition metal reduction for Ni-rich LiNi0.8Co0.2−yAlyO2 (0%, 5%, or 20% Al) layered oxide cathodes tested at 4.75 V under thermal stress (60 °C). For these tests, we compared the conventional LiPF6 salt with the more thermally stable LiBF4 salt. The CEI layers are inherently different between these two electrolyte salts, particularly for the highest level of Al-doping (20%) where a thicker (thinner) CEI layer is found for LiPF6 (LiBF4). Focusing on the aluminum environment, we reveal the type of surface aluminum species are dependent on the electrolyte salt, as Al-O-F- and Al-F-like species form when using LiPF6 and LiBF4, respectively. In both cases, we find cathode-electrolyte reactions drive the formation of a protective Al-F-like barrier at the CEI in Al-doped oxide cathodes

Atomic Structure of Surface-Densified Phases in Ni-Rich Layered Compounds.

In this work, we report the presence of surface-densified phases (β-Ni5O8, γ-Ni3O4, and δ-Ni7O8) in LiNiO2 (LNO)- and LiNi0.8Al0.2O2 (LNA)-layered compounds by combined atomic level scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). These surface phases form upon electrochemical aging at high state of charge corresponding to a fully delithiated state. A unique feature of these phases is the periodic occupancy by Ni2+ in the Li layer. This periodic Ni occupancy gives rise to extra diffraction reflections, which are qualitatively similar to those of the LiNi2O4 spinel structure, but these surface phases have a lower Ni valence state and cation content than spinel. These experimental results confirm the presence of thermodynamically stable surface phases and provide new insights into the phenomena of surface phase formation in Ni-rich layered structures

Atomic Structure of Surface-Densified Phases in Ni-Rich Layered Compounds

In this work, we report the presence of surface-densified phases (β-Ni5O8, γ-Ni3O4, and δ-Ni7O8) in LiNiO2 (LNO)- and LiNi0.8Al0.2O2 (LNA)-layered compounds by combined atomic level scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). These surface phases form upon electrochemical aging at high state of charge corresponding to a fully delithiated state. A unique feature of these phases is the periodic occupancy by Ni2+ in the Li layer. This periodic Ni occupancy gives rise to extra diffraction reflections, which are qualitatively similar to those of the LiNi2O4 spinel structure, but these surface phases have a lower Ni valence state and cation content than spinel. These experimental results confirm the presence of thermodynamically stable surface phases and provide new insights into the phenomena of surface phase formation in Ni-rich layered structures

Surface Structural and Chemical Evolution of Layered LiNi \u3c inf\u3e 0.8 Co \u3c inf\u3e 0.15 Al \u3c inf\u3e 0.05 O \u3c inf\u3e 2 (NCA) under High Voltage and Elevated Temperature Conditions

© Copyright 2018 American Chemical Society. This paper reports new insights into structural and chemical evolution of surface phases of LiNi0.8Co0.15Al0.05O2 (NCA) held at constant high voltages (up to 4.75 V) as well as high temperatures (60 °C) by correlating crystal structure using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imaging with chemistry using electron energy loss spectroscopy (EELS). We also followed the Al distribution within individual NCA particles by X-ray energy dispersive spectroscopy (EDS). The progression of these phases as a function of distance from the edge shows simultaneous evolution of crystal structures and chemistry from rocksalt to layered, forming a complete solid solution. We have also observed an extended disordered phase with rocksalt (Fm3m) symmetry in which quantitative electron energy loss spectroscopy reveals it to be an oxygen deficient cation disordered phase with chemical characteristics, as determined by EELS, similar to spinel. The formation of these disordered phases with cation and oxygen vacancies has been driven by surface oxygen loss caused by reactions with the electrolyte followed by cation migration from the octahedral 3a M (M = Ni, Co, Al) layer to the octahedral 3b Li layer. These surface rocksalt phases are not fully dense as they contain Al and Li as well as a high concentration of cation and oxygen vacancies. After discharge, Li is detected within these phases indicative that Li transport has occurred through these rocksalt phases. At 60 °C and 4.75 V a very large impedance rise is observed leading to complete cell irreversibility which is caused by significant metal dissolution from the cathode and formation of surface porosity. In the near surface region of some particles, a phase transformation from R3m (O3) to P3m1 (O1) is also observed which has become thermodynamically stable from complete delithiation as well as from local Al surface depletion

Electrochemical and Thermal Stress of LiNi0.8Co0.15Al0.05O2\mathrm{LiNi_{0.8}Co_{0.15}Al_{0.05}O_{2}} Electrodes: Evolution of Aluminum Surface Environments

For layered oxide cathodes, aluminum doping has widely been shown to improve performance, particularly at high degrees of delithiation. While this has led to increased interest in Al-doped systems, including $\mathrm{LiNi_{0.8}Co_{0.15}Al_{0.05}O_{2}}$ (NCA), the aluminum surface environment has not been thoroughly investigated. Using hard x-ray photoelectron spectroscopy measurements of the Al 1s core region for NCA electrodes, we examined the evolution of the surface aluminum environment under electrochemical and thermal stress. By correlating the aluminum environment to transition metal reduction and electrolyte decomposition, we provide further insight into the cathode-electrolyte interface layer. A remarkable finding is that Al-O coatings in LiPF$_6$ electrolyte mimic the evolution observed for the aluminum surface environment in doped layered oxides

Electrolyte-Induced Surface Transformation and Transition-Metal Dissolution of Fully Delithiated LiNi0.8Co0.15Al0.05O2.

Enabling practical utilization of layered R3̅m positive electrodes near full delithiation requires an enhanced understanding of the complex electrode-electrolyte interactions that often induce failure. Using Li[Ni0.8Co0.15Al0.05]O2 (NCA) as a model layered compound, the chemical and structural stability in a strenuous thermal and electrochemical environment was explored. Operando microcalorimetry and electrochemical impedance spectroscopy identified a fingerprint for a structural decomposition and transition-metal dissolution reaction that occurs on the positive electrode at full delithiation. Surface-sensitive characterization techniques, including X-ray absorption spectroscopy and high-resolution transmission electron microscopy, measured a structural and morphological transformation of the surface and subsurface regions of NCA. Despite the bulk structural integrity being maintained, NCA surface degradation at a high state of charge induces excessive transition-metal dissolution and significant positive electrode impedance development, resulting in a rapid decrease in electrochemical performance. Additionally, the impact of electrolyte salt, positive electrode surface area, and surface Li2CO3 content on the magnitude and character of the dissolution reaction was studied