With the rapid development of Li-ion batteries, their application to the electric vehicles (EVs) and hybrid electric vehicles (HEVs) would be expected. In recent years, Ni-based layered oxide cathode materials, such as LiNi_1/3Co_1/3Mn_1/3O₂ (NCM) and LiNi_0.8Co_0.15Al_0.05O₂ (NCA), garner considerable attention in lithium ion batteries due to their relative high energy density ^[1-2]. However, the safety concern of the batteries associated with the oxygen release, which is a large problem resulting from the instability of cathode and decomposition of electrolyte^[3-4], should be further resolved for practical application.

The layered oxides LiTMO₂ (TM=Ni, Co) become unstable in a highly delithiated state. On one hand, the extraction of more and more lithium ions gives rise to the overlapping of TM-3d band with O-2p band, which will increase the oxygen participation in the redox chemistry and causes destabilization of the lattice. On the other hand, upon oxidation (i.e. delithiation), increasing amount of unstable Ni⁴⁺(Co⁴⁺) will be formed, followed by the reduction to Ni³⁺(Co³⁺) at elevated temperatures with concurrent loss of oxygen. Meanwhile, the high voltage required for deep charging would oxidize electrolyte, thereby resulting in oxygen release and capacity fade. Therefore, the deep charging of layered oxide cathodes involving the anion oxidation would accelerate oxygen release, which seriously affects the safety of batteries. Some research efforts have been performed to enhance the safety by doping with anions^[5-6]. However, the mechanisms of redox chemistry in systems where multiple anions coexist remain unclear so far.

The stability and safety of layered-oxysulfide cathode by tuning the redox chemistry were studied based on first-principles. Since the Ni-based cathodes have more favorable prospect in the future, we chose LiNiO₂, the representative of the layered-oxide cathode, as a model in calculations. LiNiO_2-yS_y, wherein oxygen anions are partially substituted by sulfur anions, was also studied. In order to understand the role of S^2- in enhancing the stability and safety, the redox chemistry that occurs in S-substituted systems was discussed in detail. For Li_1-xNiO_1.89S_0.11, at the initial charging stage (0≤x≤0.33), the sulfur anions alongside nickel and oxygen atoms are all involved in the charge compensation, although sulfur anions contribute less to the process. In the next charging stage (x＞0.33), only nickel and oxygen atoms become oxidized owing to the donation of electrons, while sulfur anions are not involved. The partial oxidation of sulfur provides a novel line of thought in terms of designing the Li-rich layered cathode materials with charge compensation by multiple anions.

References

[1] T. Hayashi, J. Okada, E. Toda, R. Kuzuo, N. Oshimura, N. Kuwata, J. Kawamura, J. Electrochem. Soc. 2014, 161, A1007

[2] S.-K. Jung, H. Gwon, J. Hong, K.-Y. Park, D.-H. Seo, H. Kim, J. Hyun, W. Yang, K. Kang, Adv. Energy  Mater. 2014, 4, 1.

[3] K.-W. Nam, S.-M. Bak, E. Hu, X. Yu, Y. Zhou, X. Wang, L. Wu, Y. Zhu, K.-Y. Chung, X.-Q. Yang, Adv. Funct. Mater. 2013, 23, 1047.

[4] J. Hong, H.-D. Lim, M. Lee, S.-W. Kim, H. Kim, S.-T. Oh, G.-C. Chung, K. Kang, Chem. Mater. 2012, 24 2692.

[5] D.-W. Han, W.-H. Ryu, W.-K. Kim, J.-Y. Eom, H.-S. Kwon, J. Phys. Chem. C 2013, 117, 4913

[6] S.-H. Park, Y.-K. Sun, K.-S. Park, K.-S. Nahm, Y.-S. Lee, M. Yoshio, Electrochim. Acta 2002, 47, 1721