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Ni-rich Li­[Ni1–x–y Co x Al y ]­O2 (NCA) cathodes (1 – x – y = 0.8, 0.88, and 0.95) are synthesized to investigate the capacity fading mechanism of Ni-rich NCA cathodes. The capacity retention and thermal property of the cathodes deteriorate as their discharge capacity increases when the Ni fraction is increased. The capacity fading correlates well with the anisotropic volume variations caused by the H2–H3 phase transition and the resulting extent of microcracking. Although all three cathodes start to develop microcracks after being charged to 3.9 V, the potential at which microcracks propagated to the outer surface of the particle decreases with increasing Ni content. These microcracks undermine the mechanical integrity of the cathode and facilitate electrolyte penetration into the particle core, which accelerates surface degradation of the internal primary particles. Therefore, mitigating or delaying the H2–H3 phase transition is key to improving the cycling performance of Ni-rich NCA cathodes.

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With growing demands for low-cost, large-format lithium-ion batteries mainly for electric vehicles (EVs) and energy storage system (ESS), escalating price and unsustainable supply of cobalt have become increasingly acute.1 Moreover, owing to the toxic nature and an increasing concern for child labor used in ore mining, significant research is being devoted to decrease the share or completely removing cobalt from the current cathode families, particularly in high-capacity Ni-rich layered cathodes which have sufficiently high potential for commercialization.2-5 This study involves a systematic investigation on the Ni-rich Li[Ni0.9Co0.1-xMnx]O2 (x = 0, 0.05, and 0.1) layered cathodes by variation of the Mn to Co content. Remarkably, Co-free Li[Ni0.9Mn0.1]O2 (NM90) exhibits outstanding mechanical and microstructural stability resulted in excellent cycling performance, which outperformed its Co-containing Li[Ni0.9Co0.1]O2 and Li[Ni0.9Co0.05Mn0.05]O2 counterparts. Alongside observations of high voltage and high-temperature stability, Co-free NM90 demonstrate substantially higher capacity retention, increasing the attractiveness of this material for practical application. To elucidate the impact of the Mn content in Ni-rich cathodes, the characterization techniques such as in-situ XRD, TEM and micro-compression analysis as well as electrochemical measurements have conducted and provided valuable information regarding the intrinsic properties of individual cathode components and their effects on electrochemical performance of the cathode materials. References Y.-K. Sun, D. J. Lee, Y. J. Lee, Z. Chen and S.-T. Myung, ACS Appl. Mater. Interfaces 2013, 5, 11434. U.‐H. Kim, D.‐W. Jun, K.‐J. Park, Q. Zhang, P. Kaghazchi, D. Aurbach, D. T. Major, G. Goobes, M. Dixit, N. Leifer, C. M. Wang, P. Yan, D. Ahn, K.‐H. Kim, C. S. Yoon and Y.‐K. Sun, Energy Environ. Sci. 2018, 11, 1271. K.-J. Park, M.-J. Choi, F. Maglia, S.-J. Kim, K.-H. Kim, C. S. Yoon and Y.-K. Sun, Adv. Energy Mater. 2018, 8, 1703612. U.‐H. Kim, H.‐H. Ryu, J.‐H. Kim, R. Mücke, P. Kaghazchi, C. S. Yoon and Y.‐K. Sun, Adv. Energy Mater. 2019, 9, 1803902. U.‐H. Kim, L.-Y. Kuo, P. Kaghazchi, C.S. Yoon and Y.-K. Sun, ACS Energy Lett. 2019, 4, 576.

Electric vehicles (EVs) require rechargeable batteries with high energy density and excellent capacity retention to meet the target driving range and ensure sufficient vehicle service life. However, the specific energy density and lifetime of EVs are largely limited by the performance of the cathode. For this reason, research on EV batteries primarily concentrated on characterizing and developing optimal cathode materials. Among the numerous proposed cathodes, Ni-rich Li[NixCoyMn1-x-y]O2 (NCM) and Li[NixCoyAl1-x-y]O2 (NCA) cathodes are the main candidates for high performance EVs because of their high reversible capacity and operating potential. However, due to structural instability, parasitic surface reactions arising from accumulation of a NiO-like phase on the surface, it is difficult to satisfy the recommended driving range and battery lifetime at the same time.1-3 Many researches have revealed the degradation mechanism of Ni < 90%, however, there are few experimental reports on the NCM cathodes with x ≥ 0.9. Because there is a lack of comprehensive data explicitly showing the capacity fading trend of NCM cathodes (x ≥ 0.9), the current technology shows the ultimate limitation in safely utilizing the energy storage capabilities of the Ni-rich NCM cathode. In the studies presented here, Ni-rich NCM cathodes with precisely controlled compositions, LNO (x = 1.00), NCM98 (x = 0.98), NCM96 (x = 0.96), NCM94 (x = 0.94), NCM92 (x = 0.92) and NCM90 (x = 0.90) were synthesized and compared to obtain a comprehensive mechanism of the capacity fading of highly Ni-rich NCM cathodes. In situ-XRD, scanning spreading resistance microscopy (SSRM) and electrochemical analysis became the main experiment and revealed the mechanism. Reference s Yoon, C. S.; Choi, M. H.; Lim, B.-B.; Lee, E.-J.; Sun, Y.-K, Electrochem. Soc. 2015, 162, A2483-A2489. Ryu, H.-H.; Park, K.-J.; Yoon, C. S.; Sun, Y.-K, Mater. 2018, 30, 1155-1163. Yoon, C. S.; Ryu, H.-H.; Park, G.-T.; Kim, J.-H.; Kim, K.-H.; Sun, Y.-K, Mater. Chem. A, 2018, 6, 4126-4132.