Influence of Oxygen Defects on Electrochemical Performance of Ni-Rich Layered Oxides for Future Lithium-Ion Batteries

Ni-rich layered cathode active materials (CAMs) are promising candidates for application in lithium-ion batteries due to their high reversible capacity. To further improve the performance of lithium-ion batteries, changing the transition metal composition in CAMs is a good approach to gain a higher capacity, better capacity retention as well as higher rate capability and safety. The oxygen lattice has been considered and widely accepted to be electrochemically inactive, even though it plays a key role in the phase transformations that occur at high degrees of delithiation, as oxygen is released during these high delithiation states. In addition, released oxygen can cause thermal runaway, by exothermic reaction with electrolyte 1,2 . However, the effect of oxygen defects and vacancies on the electrochemical performance of cathode active materials in lithium-ion batteries has been scarcely investigated. Research has mainly focused on lithium-rich layered materials, where oxygen vacancies have led to improved battery performance in terms of rate capability, higher discharge capacity and reduced oxygen evolution 3 . However, many of these publications suffer from an unprecise content of oxygen deficiencies within the cathode active material with large errors. In this work, a solid-state reactor for electrochemical extraction of oxygen is used to obtain an accurate oxygen deficiency with low error for Ni-rich layered NCM 811 4 . Using this process, 1 mol%, 2 mol% and 5 mol% oxygen deficient NCM 811 are produced and (electro)chemically characterized. The effect of oxygen defect concentration on discharge capacity, capacity retention, rate capability and structural stability is investigated in liquid cells. In addition, electrochemical impedance spectroscopy (EIS) is used to reveal changes in electrode resistances. To obtain the cathode impedance only, a three-electrode setup is used, which utilizes a μ-reference electrode made of gold wire. References 1 R. Jung, M. Metzger, F. Maglia, C. Stinner and H. A. Gasteiger, J. Electrochem. Soc., 164, A1361 (2017). 2 R. Jung, P. Strobl, F. Maglia, C. Stinner and H. A. Gasteiger, J. Electrochem. Soc., 165, A2869 (2018). 3 B. Qiu, M. Zhang, L. Wu, J. Wang, Y. Xia, D. Qian, H. Liu, S. Hy, Y. Chen, K. An, Y. Zhu, Z. Liu, and Y. S. Meng, Nat. Commun., 7, 12108 (2016). 4 T. Nakamura, H. Gao, K. Ohta, Y. Kimura, Y. Tamenori, K. Nitta, T. Ina, M. Oishi and K. Amezawa, J. Mater. Chem. A, 7, 5009 (2019).