Surface Chemistry Dependence on Aluminum Doping in Ni-rich LiNi0.8Co0.2−yAlyO2 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.

Electrolyte reactivity at the charged Ni-rich cathode interface and degradation in Li-ion batteries

“…This indicates a chemical change in the Ni close to the NMC surface, and has previously been observed to coincide with ReSL formation during long-term cycling of NMC811, 44 potentially corresponding to formation of a Ni-F environment. 45 These observations indicate that the EC electrolyte is more reactive towards the NMC811 electrode than the other electrolytes, which is in agreement with the electrochemical data shown in Figure 1d-e.…”

Analysing trimethylaluminum infiltration into polymer brushes using a scalable area selective vapor phase process

“…Small peaks at approximately 1561.0 eV and 1564.3 eV are attributed Al sub oxide 67 and surface based, Al-O like bonding, respectively. 68 Negligible difference in Al concentration is observed between the two pyridine structures. Although the Al intake is slower initially with the P2VP brush, similar amounts of Al reside in both PVPs when sufficient cycles are completed.…”

Hard x-ray photoelectron spectroscopy: a snapshot of the state-of-the-art in 2020

“…Laboratory-based HAXPES systems have exploited the accessibility of additional, deeper core levels to study depth-dependent phenomena in both bulk as well as multilayer systems. [85][86][87][88][89][90][91] A general advantage not just of laboratory systems, but any HAXPES experiment, is the ability to access deeper core levels opening up new experimental and analytical strategies. [43,[92][93][94][95] Figure 4 gives a schematic overview of core levels of transition metals and lanthanides that become available when moving A c c e p t e d M a n u s c r i p t…”

“…[96,97] Another example that has been explored with laboratory HAXPES is in complex materials with multiple elements present that often have overlapping core levels in the soft X-ray range, such is the case for LiNi 0.8 Co 0.2-y Al y O layered oxide cathodes for batteries. [85] Probing the Al 1s core level instead of the lower energy Al 2s and 2p states excludes spectral contamination from any of the other elements present.…”

“…[89] Several of the studies mentioned here also exemplify the complementarity of laboratory-and synchrotron-based HAXPES systems to enable a comprehensive and efficient exploration of samples of interest. [85,87,89,106] In contrast to synchrotron-or FEL-based HAXPES, the excitation energy in laboratory-based HAXPES systems is fixed to the characteristic X-ray emission of the anode material. This has important implications for the information depth (see general discussion on this topic in Section 3.1.1.…”