Nickel-rich layered lithium transition metal oxides (LiNi 1−x−y Co x Mn y O 2 and LiNi 1−x−y Co x Al y O 2 , x + y ≤ 0.2) are the most attractive cathode materials for the next generation lithium-ion batteries for automotive application. However, they suffer from structural/interfacial instability during repeated charge/discharge, resulting in severe performance degradation and serious safety concerns. This work provides a comprehensive review about challenges and strategies to advance nickel-rich layered cathodes specifically for harsh (high-voltage, hightemperature, and fast charging) operations. Firstly, the degradation pathways of nickel-rich cathodes including surface/interface degradation, undesired cathode-electrolytes parasitic reactions, gas evolution, inter/intragranular cracking, and electrical/ionic isolation are discussed. Then, recent achievements in stabilizing the structure/interface of nickel-rich cathodes via surface coating, cation/anion doping, composition tailoring, morphology engineering, and electrolytes optimization are summarized. Moreover, challenges and strategies to improve the performance of Ni-rich cathodes at the electrode level are discussed. Outlook and perspectives to promote the practical application of nickel-rich layered cathodes toward automotive application are provided as well.

Oxygen redox at high-voltage has emerged as a transformative paradigm for high-energy battery cathodes by offering extra capacity beyond conventional transition-metal redox. However, it suffers from voltage hysteresis, voltage fade, and capacity drop upon cycling. Here, we show that, by eliminating the domain boundaries in the often-considered single-crystalline battery particles, layered oxide cathodes demonstrate exceptional capacity and voltage stability during high-voltage operation. Our combined experimental and theory studies for the first time reveal that the elimination of domain boundaries could enhance the reversible lattice oxygen redox while inhibiting the irreversible oxygen release, leading to significantly suppressed structural degradation and improved mechanical integrity during battery cycling and abuse heating. The robust oxygen redox enabled through domain boundary control provides practical opportunities towards high-energy, long-cycling, and safe batteries. MainHigh-energy batteries rely on high-capacity and high-voltage operation of the cathodes.Fundamentally, the capacity of a transition metal oxide-based cathode is determined by the amount of active Li, while the voltage is defined by the redox reactions affected by structural configurations 1 . This has led to two associated trends in recent cathode development: Li-excess compounds due to the large amount of Li (capacity) and oxygen redox (OR) at high-voltage 2-6 .However, intensive studies have shown that OR activities can trigger detrimental structural effects such as oxygen release, surface reactions, and phase transition, leading to severe voltage hysteresis, voltage fade, and poor capacity stability 3,5 . Despite extensive mechanistic understanding 3,5 and material optimization such as structural control [7][8][9] , chemical composition manipulation 10 , and cationic/anionic doping [11][12][13] , the fundamental origin of the OR instability remains under active debate, and the practical control of high-voltage operation involving OR remains a formidable challenge. The key relies on a strategy that enhances the reversible OR in the lattice, while suppressing or even eliminating other detrimental oxygen activities.Theoretical studies suggest that the surface could favour the migration of oxygen ions and promote the formation of oxygen vacancies due to the open atomic structure [14][15][16] . Therefore, surface-initiated irreversible oxygen loss and the associated structural transformation have long been considered as the root cause of the capacity decay and voltage fade of Li-excess layered cathodes when activating the OR process [16][17][18] . However, surface coatings to mitigate the oxygen loss have proven insufficient to achieve a fully reversible OR 5,18,19 .Grain boundaries (GBs), the surface that separates individual grains from each other, play a vital role in materials' properties. In layered oxide cathodes, the GBs have been predominantly referred to the boundaries between primary particles of polycrystalline cathodes, while ...

The further improvement of sodium ion batteries requires the elucidation of the mechanisms pertaining to reversibility, which allows the novel design of the electrode structure. Here, through a hydrogel-embedding method, we are able to confine the growth of few-layer SnS2 nanosheets between a nitrogen- and sulfur-doped carbon nanotube (NS-CNT) and amorphous carbon. The obtained carbon-sandwiched SnS2 nanosheets demonstrate excellent sodium storage properties. In operando small-angle X-ray scattering combined with the ex situ X-ray absorption near edge spectra reveal that the redox reactions between SnS2/NS-CNT and the sodium ion are highly reversible. On the contrary, the nanostructure evolution is found to be irreversible, in which the SnS2 nanosheets collapse, followed by the regeneration of SnS2 nanoparticles. This work provides operando insights into the chemical environment evolution and structure change of SnS2-based anodes, elucidating its reversible reaction mechanism, and illustrates the significance of engineered carbon support in ensuring the electrode structure stability.