Lithium-ion batteries (LIBs) have become a high-density energy source for portable electronic devices and small home appliances. However, LIBs are beset with significant challenges for replacing gasoline-powered automobiles with fully electric vehicles (EVs), which are becoming increasingly necessary to achieve a sustainable mode of personal transportation. One of the most difficult technological hurdles for EV batteries is increasing the specific energy density, which dictates the drive range of EVs per charge.¹ LIBs based on layered transition-metal oxides, Li[Ni_xCo_yMn_1−x−y]O₂ (NCM), have long been considered as an alternative cathode for EVs and the main strategy for increasing the discharge capacity of NCM cathodes has been a progressive increase of the Ni fraction.^1,2 However, it causes a proportional deterioration of the cathode’s ability to retain its original capacity during cycling.³ The observed capacity fading of NCM cathodes is typically attributed to the parasitic surface reactions arising from accumulation of a NiO-like phase on the surface of the cathode material and oxygen release that destabilizes the crystal structure.^2,4 Anisotropic lattice contraction during cycling, which is the main capacity fading mechanism for LiNiO₂, can also contribute to structural degradation of NCM cathodes.⁵ A comprehensive study of the capacity fading mechanisms of NCM cathodes for a wide range of Ni-rich compositions (i.e., Li[Ni_0.6Co_0.2Mn_0.2]O₂, Li[Ni_0.8Co_0.1Mn_0.1]O₂, Li[Ni_0.9Co_0.05Mn_0.05]O₂, and Li[Ni_0.95Co_0.025Mn_0.025]O₂) was investigated by examinging the relative effects of the bulk structural degradation (anisotropic volume change) and the surface degradation (formation of an inactive and resistance-increasing surface layer) during cycling. The electrochemical data for the NCM cathode series are correlated to the bulk and surface structural changes determined by X-ray diffraction and transmission electron microscopy.

References

1. S.-T. Myung, F. Maglia, K.-J. Park, C. S. Yoon, P. Lamp, S.-J. Kim, Y.-K. Sun, ACS Energy Lett. 2017, 2, 196.
2. D.-W. Jun, C. S. Yoon, U.-H. Kim, Y.-K. Sun, Chem. Mater. 2017, 29, 5048-5052.
3. H.-J. Noh, S. Youn, C. S. Yoon, Y.-K. Sun, J. Power Sources 2013, 233, 121-130.
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5. J.-M. Lim, T. Hwang, D. Kim, M.-S. Park, K. Cho, M. Cho, Sci. Rep. 2017, 7, 39669.