A series of single-crystal, Ni-rich Li­[Ni x Co y Mn1–x–y ]­O2 (NCM) cathodes (x = 0.7, 0.8, and 0.9) with particle diameters of ∼3 μm are systematically compared with polycrystalline cathodes with corresponding Ni contents. Despite their high resistance to microcracking, the electrochemical performances of single-crystal NCM cathodes, in terms of capacity and cycling stability, are inferior to those of polycrystalline NCM cathodes. In situ XRD and TEM analyses reveal that the lithium concentrations in single-crystal NCM cathodes become spatially inhomogeneous during cycling; this phenomenon is exacerbated by high C rates and Ni contents, resulting in the coexistence of phases with widely different unit cell dimensions within a single cathode particle. This coexistence of two phases induces nonuniform stress that generates structural defects, impairing the diffusion of lithium ions and, eventually, leading to rapid capacity fading.

The ultrafine-grained Ni-enriched Li[Ni0.95Co0.04Mo0.01]O2 (NCMo95) cathode achieved by inhibiting particle coarsening imparts the necessary mechanical toughness and significantly extends the battery life.

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Recharging capability of Ni-rich layered cathodes deteriorates rapidly upon cycling mainly from mechanical instability caused by removing a large amount of Li ions from the host structure. Through multi-stage microstructural...

of the global EV market, Li-ion battery (LIB) technology has progressed to meet the demand for longer driving ranges and extended service lives, while increasing the thermal stability and reducing the cost of the battery. As the performance of LIBs is largely determined by the cathode material, the development of high-performance LIBs for EVs has focused on increasing the capacity of the cathode by usingHowever, cathodes with a high Ni content suffer from inherent structural and chemical instabilities, which lead to rapid capacity fading and thermal instability. [3][4][5][6][7][8][9] In particular, the rapid capacity fading of Ni-rich layered cathodes is largely caused by microcracking and the resultant microstructural instability. [9][10][11][12][13][14][15][16][17][18][19] Microcracks create channels through which the electrolyte can infiltrate the cathode particles, which increases the surface area exposed to electrolyte and worsens parasitic electrolyte attack. The subsequent degradation of the internal surfaces accelerates the accumulation of NiO-like impurity layers at the cathode-electrolyte interface, which hinders electrochemical reactions. [9,[19][20][21][22][23][24][25] As microcracking is caused by abrupt contraction of the unit cells during the H2-H3 phase transition at a high state of charge (SoC), microstructural engineering to facilitate the dissipation of internal mechanical strain and mitigate microcracking in the cathode particles has been extensively investigated. [19,21,[24][25][26][27][28][29][30] One such engineered microstructure is a highly oriented microstructure in which elongated primary particles are aligned along the radial direction in the secondary particle periphery. This radial alignment of rod-shaped primary particles effectively dissipates localized strain by allowing the unit cell to contract and expand uniformly. Microstructural manipulation can be achieved by compositional gradient design of transition metal ions [19,24,25,31] or by doping with various atoms (e.g., B, Ta, Mo, W, and Sb) during lithiation. [18,[26][27][28][29][30]32,33] Furthermore, a reduction in volume deformation and internal stress inside the particle via Al doping can enhance the structural integrity and robustness of cathode. [30,34,35] In addition to microcracking, the time during which highly reactive Ni 4+ ions are exposed to the electrolyte when the cathode is in a highly charged state also affects the deterioration of the cathode. Although the exposure time is not Li-ion batteries (LIBs) in electric vehicles (EVs) are usually operated intermittently and maintained at high states of charge (SoCs) for long periods. Because the internal particles of Ni-rich cathodes are easily exposed to the electrolyte at high SoCs owing to mechanical instability, the electrolyte exposure time-during which highly reactive Ni 4+ ions react with the electrolyte-critically affects the degradation of the cathode. Here, 1 mol% B doping of a core-shell concentration gradient (CSG) Li[Ni 0.88 Co 0.10 Al 0.02 ]O 2 cathode (C...