Electrolyte-Induced Surface Transformation and Transition-Metal Dissolution of Fully Delithiated LiNi_0.8Co_0.15Al_0.05O₂

Abstract

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[NiCoAl]O (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 LiCO content on the magnitude and character of the dissolution reaction was studied.

Surface/Interface Structure Degradation of Ni‐Rich Layered Oxide Cathodes toward Lithium‐Ion Batteries: Fundamental Mechanisms and Remedying Strategies

“…When the Ni content further increases to more than 80 mol%, for instance, the dissolution of TM from the LiNi 0.87 Co 0.09 Mn 0.04 O 2 cathode and their depositions on graphite anode as well as corresponding reactions with the electrolyte with the high‐resolution chemical composition analysis are studied ( Figure a), revealing that the TM dissolution preferentially occurs for the particles with large surface area/volume ratios due to the high exposure of the particle surface to electrolytes. As for the Ni‐rich NCA cathode, the investigation performed by the Faenza et al affirms the structural decomposition and TM dissolution occurring on the NCA surface at full delithiation by virtue of operando microcalorimetry and electrochemical impedance spectroscopy, and the excessive TM dissolution induced by the surface degradation at a high SOC (Figure b), leading to the huge increase in impedance and rapid decrease in electrochemical properties. Moreover, the dissolution of elemental Co and Ni occurs in the stoichiometric ratio, which is well consistent with the original stoichiometry in the parent NCA …”

“…As for the Ni‐rich NCA cathode, the investigation performed by the Faenza et al affirms the structural decomposition and TM dissolution occurring on the NCA surface at full delithiation by virtue of operando microcalorimetry and electrochemical impedance spectroscopy, and the excessive TM dissolution induced by the surface degradation at a high SOC (Figure b), leading to the huge increase in impedance and rapid decrease in electrochemical properties. Moreover, the dissolution of elemental Co and Ni occurs in the stoichiometric ratio, which is well consistent with the original stoichiometry in the parent NCA …”

“…Copyright 2018, Elsevier Ltd. b) Illustration for the electrolyte‐induced surface transformation and TM dissolution in the fully delithiated NCA. Reproduced with permission . Copyright 2017, American Chemical Society.…”

Surface Structural and Chemical Evolution of Layered LiNi0.8Co0.15Al0.05O2 (NCA) under High Voltage and Elevated Temperature Conditions

“…The detailed results of the respective changes in electrochemistry are reported in a companion paper. 50 Two cells were tested at each voltage and Figure 3b). This alternative occupancy of the Li sites is a characteristic feature of the spinel phase, which can be clearly identified by HAADF-STEM.…”

Editors' Choice—Growth of Ambient Induced Surface Impurity Species on Layered Positive Electrode Materials and Impact on Electrochemical Performance

“…Our previous work demonstrates that layered oxide materials with higher contents of surface impurities have an amplified propensity for transition metal dissolution, likely caused by the increase of reduced transition metals that are present at the material's surface. 33 Impurity species other than Li 2 CO 3 can also have adverse effects on a cell's performance. For example, it is well known that adsorbed and absorbed H 2 O will react with fluorinated electrolyte salts to produce HF, which then degrades the positive electrode.…”