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The ever‐increasing energy density requirements in electric vehicles (EVs) have boosted the development of Ni‐rich layered oxide cathodes for state‐of‐the‐art lithium‐ion batteries. Nevertheless, the commercialization of polycrystalline Ni‐rich cathodes (PCNCs) is hindered by the severe performance degradation and safety concerns that are tightly related to its particle cracking during cycling. Single‐crystalline Ni‐rich cathodes (SCNCs) with eliminated grain boundaries and high mechanical strength have recently attracted extensive attention owing to their superior structural and cycling stability, which present high crack resistance during electrochemical operation. Various articles have focused on the trial‐and‐error synthesis and modifications of SCNCs, as well as the comparison of performances and mechanisms with PCNCs. However, there has been much less effort in systematic analysis and summary to reveal their key challenges, controversies, and the corresponding primary causes. In this review, the advantages and debates in structural and electrochemical properties of SCNCs over PCNCs are summarized to provide fundamental understanding of SCNCs. Then the current practical issues and challenges are comprehensively discussed from the viewpoints of both academia and industry, as well as the proposed modification strategies and underlying mechanisms for SCNCs. The outlook and perspectives are further given to facilitate the commercial applications of SCNCs in high‐performance EVs.

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one important strategy is to prepare micrometer-sized secondary particles composed of agglomerated nanosized primary particles. However, abrupt anisotropic lattice shrinkage occurs along the crystallographic c-axis derived from phases transitions (H2 → H3) at high states of charge, potentially resulting in the generation and buildup of substantial mechanical stress, which is the root cause of microcracks formation, eventual pulverization and electrical isolation of the secondary particles during prolonged cycling. [2] Note that intragranular microcracking has deleterious effects on the performances of nickel-rich cathode, which might provide fast channel for electrolyte infiltration into the particle interior, accelerating parasitic side reactions at the internal electrode/electrolyte interface and associated accumulation of NiO-like detrimental phase and continuous impedance growth. [2b,3] What's worse, a series of other degradation processes could in turn be triggered by this chain-reaction mode. For example, H + species in the electrolyte would severely corrode the surface of reactive particles, giving rise to the dissolution of transition metal (TM) ions, coupled with the release of oxygen, and it further induces profound voltage attenuation and negative safety issues. [3a,4] Therefore, the massive applications of nickel-rich cathodes mainly rely on addressing these bottleneck problems.The discussions above highlight the demand to prevent both physical and chemical degradation of Ni-rich cathodes. Element doping in the bulk phase has been proven to be one of the most effective remedies to inhibit problematic microcracks and stabilize the crystal structure of Ni-enriched layered cathodes. [3b,c,5] In recent years, various ion dopants, such as B, [3b,5a,6] Zr, [5b,c] W, [5d,e] Nb, [5f ] Mg, [5g,h] Ta, [5c] Ce, [5i] Mo, [5j] and Al [5k,7] etc., have been reported to arrest the pernicious effect. For example, Hong and co-workers incorporated element B into the crystal structure of Ni-rich cathode, which remarkably alleviates the inherent microcracking problem. [6] With boron ions introduced into the microstructure, the randomly oriented elongated polygonal primary particles could be transformed into highly textured plate-like microstructures distributed along the radial direction, significantly releasing the interior mechanical strain in the highly delithiated state. Thereby the nucleation and Capacity fading and safety concerns accompanied other deep-rooted challenges have severely hindered commercial development of Ni-rich layered cathodes. Herein, a robust Sr-doped Ni-rich cathode is structurally designed by the reconstruction of the crystal lattice and electronic distribution. Notably, the orbital hybridization between Ni 3d (t 2g ) and O 2p is remarkably reinforced owing to the shortened NiO bond enabled by the electrostatic interaction between Ni and Sr atoms, giving rise to the enhanced crystal structure. Theoretically, the formation energy of oxygen vacancies is greatly increased due to the...

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LiCoO2 (LCO) is widely applied in today's rechargeable battery markets for consumer electronic devices. However, LCO operations at high voltage are hindered by accelerated structure degradation and electrode/electrolyte interface decomposition. To overcome these challenges, co‐modified LCO (defined as CB‐Mg‐LCO) that couples pillar structures with interface shielding are successfully synthesized for achieving high‐energy‐density and structurally stable cathode material. Benefitting from the “Mg‐pillar” effect, irreversible phase transitions are significantly suppressed and highly reversible Li+ shuttling is enabled. Interestingly, bonding effects between the interfacial lattice oxygen of CB‐Mg‐LCO and amorphous CoxBy coating layer are found to elevate the formation energy of oxygen vacancies, thereby considerably mitigating lattice oxygen loss and inhibiting irreversible phase transformation. Meanwhile, interface shielding effects are also beneficial for mitigating parasitic electrode/electrolyte reactions, subsequent Co dissolution, and ultimately enable a robust electrode/electrolyte interface. As a result, the as‐designed CB‐Mg‐LCO cathode achieves a high capacity and excellent cycle stability with 94.6% capacity retention at an extremely high cut‐off voltage of 4.6 V. These findings provide new insights for cathode material modification methods, which serves to guide future cathode material design.

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Sodium layered oxides always suffer from sluggish kinetics and deleterious phase transformations at deep-desodiation state (i.e., >4.0 V) in O3 structure, incurring inferior rate capability and grievous capacity degradation. To tackle these handicaps, here, a configurational entropy tuning protocol through manipulating the stoichiometric ratios of inactive cations is proposed to elaborately design Na-deficient, O3-type Na x TmO2 cathodes. It is found that the electrons surrounding the oxygen of the TmO6 octahedron are re­arranged by the introduction of MnO6 and TiO6 octahedra in Na-deficient O3-type Na0.83­Li0.1­Ni0.25­Co0.2­Mn0.15­Ti0.15­Sn0.15­O2−δ (MTS15) with expanded O–Na–O slab spacing, giving enhanced Na+ diffusion kinetics and structural stability, as disclosed by theoretical calculations and electrochemical measurements. Concomitantly, the entropy effect contributes to the improved reversibility of Co redox and phase-transition behaviors between O3 and P3, as clearly revealed by ex situ synchrotron X-ray absorption spectra and in situ X-ray diffraction. Notably, the prepared entropy-tuned MTS15 cathode exhibits impressive rate capability (76.7% capacity retention at 10 C), cycling stability (87.2% capacity retention after 200 cycles) with a reversible capacity of 109.4 mAh g–1, good full-cell performance (84.3% capacity retention after 100 cycles), and exceptional air stability. This work provides an idea for how to design high-entropy sodium layered oxides for high-power density storage systems.