Ni-rich layered oxides (LiNi_xCo_yMn_zO₂, x≥0.8, x+y+z=1) would fulfil the energy density requirements of the automobile industry since they offer outstanding capacities at relatively high mean voltages and sufficient power densities^1-3. However, the materials still show significant capacity and voltage fade which requires substantial research. Recently we have shown that the electronic structure is the key to understand the performance but also the failure of the materials¹. Thus, we intend to modify the electronic structure using anionic dopants such as Boron. Thus, homogenous B-substituted NCM811s (BNCM811_x%, x% means the x at. % of Boron) are synthesized and the materials reveal a significant change in the electronic structure as evident from x-ray photoelectron spectroscopy (XPS) and near edge x-ray absorption spectroscopy (NEXAFS). Interestingly, a shift of the so called H₂-H₃ peak⁴ to higher potentials was observed by Boron substitution and BNCM811s_2% shows relative higher initial discharge capacity (Figure 1A.) at a slightly higher mean voltage, but a lower cycling stability compared to NCM811 (Figure 1B.).

The H₂-H₃ differential capacity peak (Figure 1C) might also include oxygen release⁵, which is one of the main reasons restricting the cycling stability. Near edge x-ray absorption spectroscopy (NEXAFS) shows that the H₂-H₃ peak corresponds to a reaction from Ni³⁺ to Ni²⁺ (upon charge) and Ni²⁺ to Ni³⁺ (upon discharge) which suggest a reaction like NiO₂ ⥂ NiO + ½ O₂ as the underlying process. At the same time, an Oxygen K peak at 531 eV appears in the NEXAFS spectra, which was assigned to O-O formation in the host structure of Li-rich materials^{6, 7}. Note that the electronic structure of the materials was determined in 5 mAh/g steps over the H₂-H₃ peak and due to the high resolution, this process became visible for the first time. The findings suggest that dimer formation is not only a phenomenon of Li-rich materials but can also be found in Ni-rich layered oxides at high states of charge.

The origin of dimer formation and oxygen release can be traced back to the electronic configuration of Ni. Charge transfer multiplet calculations reveal, that Ni²⁺ reacts to covalent Ni³⁺ upon charge (and vice versa upon discharge)¹. Ni³⁺ has a 3d⁷ electronic state, whose low spin configuration is more preferred than its high spin configuration. Thus, Ni³⁺ is more prone to Jahn-Teller (JT) distortions and tends to form covalent bonds. Consequently, the electrons are more bound to the individual sites making a further oxidation of the transition metal almost impossible. In conclusion, Boron substitution helps us to understand the function and failure of layered oxides in Li-ion batteries on an atomistic scale and the findings can be used as a design guide for future materials.

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3. S.-J. Yoon, K.-J. Park, B.-B. Lim, C. S. Yoon and Y.-K. Sun, Journal of the Electrochemical Society, 2014, 162, A3059.
4. S. Jamil, G. Wang, L. Yang, X. Xie, S. Cao, H. Liu, B. Chang and X. Wang, Journal of Materials Chemistry A, 2020, 8, 21306-21316.
5. K. Märker, P. J. Reeves, C. Xu, K. J. Griffith and C. P. Grey, Chemistry of Materials, 2019, 31, 2545-2554.
6. E. Hu, X. Yu, R. Lin, X. Bi, J. Lu, S. Bak, K.-W. Nam, H. L. Xin, C. Jaye and D. A. Fischer, Nature Energy, 2018, 3, 690-698.
7. K. Kleiner, B. Strehle, A. R. Baker, S. J. Day, C. C. Tang, I. Buchberger, F.-F. Chesneau, H. A. Gasteiger and M. Piana, Chemistry of Materials, 2018, 30, 3656-3667.