Challenges and approaches of single-crystal Ni-rich layered cathodes in lithium batteries

Abstract

High-energy density and high safety are incompatible with each other in lithium battery, which challenges today's energy storage and power applications. Ni-rich layered transition metal oxides (NMC) have been identified as the primary cathode candidate for powering next-generation electric vehicles and were extensively studied in the last two decades, leading to the fast promotion on its market share including both polycrystalline and single crystal NMC cathodes. Compared to polycrystalline NMC, single crystal NMC appears to be the superior one, especially at low Ni content (≤ 60%). However, the Ni-rich single crystal NMC cathodes experience even faster capacity decay than the polycrystalline one, rendering them unsuitable for practical applications. Accordingly, this work will systematically review the attenuation mechanism of single crystal NMC and generate fresh insights into the valuable research pathway. This perspective is desired to provide an accurate direction for the development of Ni-rich single crystal NMC cathodes.

Atomic-Resolution In Situ Exploration of the Phase Transition Triggered Failure in a Single-Crystalline Ni-Rich Cathode

“…This kind of SRL formed by disordered cations often occurs in Ni-rich cathode materials. For commercial large-scale production of single-crystal NMC, high-temperature synthesis is the simplest and most commonly used method . A high temperature not only promotes the growth of single crystals but also leads to thermodynamic instability of the cathode surface .…”

“…For commercial large-scale production of single-crystal NMC, high-temperature synthesis is the simplest and most commonly used method. 43 A high temperature not only promotes the growth of single crystals but also leads to thermodynamic instability of the cathode surface. 44 During the annealing process, there is a driving force that drives Li to leave the oxide lattice, resulting in the loss of Li on the surface 44 and the generation of Li vacancies.…”

Microstructures of layered Ni-rich cathodes for lithium-ion batteries

“…As there is no grain boundary nor intergranular cracks in SC particles, less surface degradation, electrolyte decomposition, and transition metal dissolution will be involved during the operation, leading to excellent cycle stability. 260 Yong Yang and co-workers 247 synthesized the SC LiNi 0.83 Co 0.11 Mn 0.06 O 2 (SC-NCM) material with a diameter of 3–6 μm, and the corresponding cathode exhibits significantly improved capacity retention after long cycles, at both room temperature (Fig. 13n) and 45 °C (Fig.…”

Unrecoverable lattice rotation governs structural degradation of single-crystalline cathodes

“…Single-crystalline NMCs (SC-NMCs), with the absence of intergrain boundaries and with enhanced mechanical properties, are anticipated to address the aforementioned problems ( 10 – 12 ). In reality, however, SC-NMCs with Ni content ≥70% experience lower practical capacity and rapid capacity degradation, despite their high resistance to microcrack formation ( 13 – 15 ). Several prevailing hypotheses have been proposed to explain the degradation mechanism of SC-NMCs, including irreversible structure transition and slabs gliding–induced microcracking ( 5 , 16 ); yet, the true cause remains debatable.…”

“…Irreversible structure transitions are often undetected from the macroscale or bulk sensitive characterizations and are only observed within a few nanometers from the particle surface through transmission electron microscopy (TEM) ( 10 , 17 ). Additionally, more-severe and irreversible structure transition was observed in polycrystalline NMCs ( 18 , 19 ), despite showing less capacity degradation ( 15 ). This calls into question whether surface structure reconstruction dominates the capacity fade in single-crystal cathodes.…”