Facet‐dependent Thermal and Electrochemical Degradation of Lithium‐rich Layered Oxides

Abstract

Lithium‐rich layered oxides (LLOs) are promising candidate cathode materials for safe and inexpensive high‐energy‐density Li‐ion batteries. However, oxygen dimers are formed from the cathode material through oxygen redox activity, which can result in morphological changes and structural transitions that cause performance deterioration and safety concerns. Herein, a flake‐like LLO is prepared and aberration‐corrected scanning transmission electron microscopy (STEM), in situ high‐temperature X‐ray diffraction (HT‐XRD), and soft X‐ray absorption spectrum (sXAS) are used to explore its crystal facet degradation behavior in terms of both thermal and electrochemical processes. Void‐induced degradation behavior of LLO in different facet reveals significant anisotropy at high voltage. Particle degradation originates from side facets, such as the (010) facet, while the close (003) facet is stable. These results are further understood through ab initio molecular dynamics calculations, which show that oxygen atoms are lost from the {010} facets. Therefore, the facet degradation process is that oxygen molecular formed in the interlayer and accumulated in the ab plane during heating, which result in crevice‐voids in the ab plane facets. The study reveals important aspects of the mechanism responsible for oxygen ‐anionic activity‐based degradation of LLO cathode materials used in lithium‐ion batteries. In particular, this study provides insight that enables precise and efficient measures to be taken to improve the thermal and electrochemical stability of an LLO.

Comparative Study of High-Temperature Cycling and Thermal Stability of LLOs and NCMs under Medium–High Voltage

“…What’s more, the slope of the curve in the image represents the speed of the exothermic reaction. A relatively gentle reaction peak indicates a longer reaction time, which also means that the battery control system can be given a longer reaction time to avoid disasters . The reaction peaks of LLO-1 and LLO-2 are milder.…”

“…A relatively gentle reaction peak indicates a longer reaction time, which also means that the battery control system can be given a longer reaction time to avoid disasters. 48 The reaction peaks of LLO-1 and LLO-2 are milder. Results show that the thermal runaway of LLOs can only occur at very high temperature, and the reaction is very gentle and easy to control.…”

Facet-Dependent Ni Segregation in a Micron-Sized Single-Crystal Li1.2Ni0.2Mn0.6O2 Cathode

“…Single-crystal cathodes are emerging as a promising material class for Li-ion batteries, − since they exhibit appealing electrochemical properties: (1) higher capacity retention and longer cycle life; , (2) selectively exposing crystal facets for the controlled cathode–electrolyte interphase (CEI) and surface reconstruction during cycling; , (3) alleviating particle fracture during cycling; and (4) fast charging . In particular, different electrochemical characteristics such as Li + transport, cathode–electrolyte interactions, surface strain state, and thermal stability have been found to be directly dependent on the facet formation and surface reconstructed phases in layered cathodes. ,− For instance, the facets arranged in a mix of cations and anions and surface reconstructed high-voltage spinel phase are more favorable for Li + ion diffusion. , The 102 facet is comparatively less reactive with a LiPF 6 -based liquid electrolyte and tends to form a Li x PO y F z -rich CEI layer . Surface strain is more pronounced in the facets arranged in a mix of cations and anions, which results in the formation of specific surface phases and fractures caused by cycling. , Meanwhile, the 003 facet is more stable at elevated temperatures. , Therefore, understanding the phase formation and distribution in the micrometer-sized Li 1.2 Ni 0.2 Mn 0.6 O 2 single crystals becomes very critical now more than ever.…”

“…25,26 Meanwhile, the 003 facet is more stable at elevated temperatures. 27,28 Therefore, understanding the phase formation and distribution in the micrometer-sized…”

Surface Facet Dependent Cycling Stability of Layered Cathodes

“…[2,3] Moreover, the intrinsic surface properties, such as particle shape and size, are also well-known to influence the electrochemical performance. [4][5][6][7] Particularly, surface facet is found to influence oxygen release, [8] thermal decomposition, [8,9] and surface structure reconstruction in layered cathodes. [10][11][12][13] The surface facet of different crystal indices can also influence the ion transportation kinetics.…”