(Battery Division Student Research Award Address Sponsored by Mercedes-Benz Research & Development) Towards Understanding and Controlling Bulk Anionic Redox in Li-ion Positive Electrode Materials Through Electronic Structure Tuning

Abstract

Positive electrode materials such as layered lithium nickel, manganese and cobalt oxides (NMC)1 ,2 in Li-ion batteries typically store charge by relying on the redox activity of transition metal species, which is accompanied by the intercalation and deintercalation of Li ions into and out the host structure. Anionic redox in positive electrode materials can provide additional charge storage beyond the conventional metal redox. However, the physical origin of observed anion redox as well as the requirements for the reversible anionic redox activity remain under debate, hindering rational design of new electrode materials leveraging reversible anionic redox.In this talk, we first focus on understanding the cationic and anionic redox process in the positive electrode materials upon lithium deintercalation using X-ray absorption and emission spectroscopy (XAS and XES), X-ray photoelectron spectroscopy (XPS), coupled with density functional theory (DFT) calculations. We show electronic signatures of oxygen-oxygen coupling, direct evidence central to lattice oxygen redox (O2-/(O2)n-), in charged Li2-xRuO3 after Ru oxidation (Ru4+/Ru5+) upon first-electron removal with lithium de-intercalation. This lattice oxygen redox of Li2-xRuO3 was accompanied by bulk Ru reduction.3 This observed redox trend is in stark contrast of the observations in charged Ni-rich NMC upon charging. In Ni-rich NMC positive electrodes, nickel oxidation is primarily responsible for the charge capacity up to removing ~0.7 Li, beyond which is followed by Ni reduction near the surface (up to 100 nm) due to oxygen release, where there is no significant bulk metal reduction observed.4 The uniqueness of Ru-based system lies in the highly covalent nature of Ru-O bond, which stabilizes the O2-/(O2)n- intermediates, forbidding further oxygen release.We further discuss that a strong metal-oxygen covalency is needed to enhance a reversible anionic redox.5 Differential electrochemical mass spectrometry (DEMS) was employed to monitor the oxygen release and quantify the reversibility of anionic redox of Li2Ru0.75 M 0.25O3 (M= Ti, Cr, Mn, Fe, Ru, Sn, Pt, Ir) upon first charge. We show through X-ray absorption spectroscopy that more ionic substituents and reduced metal-oxygen covalency introduce irreversible oxygen redox, accompanied with easier distortion of M-O octahedron and smaller barrier for forming oxygen dimer within the octahedron. We propose a universal electronic structure descriptor that could control bulk anionic redox, leading to a rational design strategy to enhance the cycling stability of Ni-rich NMC6 as well as Li-rich positive electrodes. Our study has laid a solid foundation for future high-throughput screening of novel and affordable metal oxides for battery and electrocatalysis applications. Reference (1) Noh, H.-J.; Youn, S.; Yoon, C. S.; Sun, Y.-K. Comparison of the Structural and Electrochemical Properties of Layered Li [NixCoyMnz] O2 (X= 1/3, 0.5, 0.6, 0.7, 0.8 and 0.85) Cathode Material for Lithium-Ion Batteries. Journal of power sources 2013, 233, 121–130.(2) Jung, R.; Metzger, M.; Maglia, F.; Stinner, C.; Gasteiger, H. A. Oxygen Release and Its Effect on the Cycling Stability of LiNixMnyCozO2 (NMC) Cathode Materials for Li-Ion Batteries. Journal of The Electrochemical Society 2017, 164 (7), A1361–A1377.(3) Yu, Y.; Karayaylali, P.; Nowak, S. H.; Giordano, L.; Gauthier, M.; Hong, W.; Kou, R.; Li, Q.; Vinson, J.; Kroll, T. Revealing Electronic Signatures of Lattice Oxygen Redox in Lithium Ruthenates and Implications for High-Energy Li-Ion Battery Material Designs. Chemistry of Materials 2019, 31 (19), 7864–7876.(4) Yu, Y.; Karayaylali, P.; Giordano, L.; Corchado-García, J.; Hwang, J.; Sokaras, D.; Maglia, F.; Jung, R.; Gittleson, F. S.; Shao-Horn, Y. Probing Depth-Dependent Transition-Metal Redox of Lithium Nickel, Manganese, and Cobalt Oxides in Li-Ion Batteries. ACS Applied Materials & Interfaces 2020.(5) Yu, Y.; Karayaylali, P.; Sokaras, D.; Giordano, L.; Kou, R.; Sun, C.-J.; Maglia, F.; Jung, R.; Gittleson, F. S.; Shao-Horn, Y. Towards Controlling the Reversibility of Anionic Redox in Transition Metal Oxides for High-Energy Li-Ion Positive Electrodes. Energy & Environmental Science 2021, 14 (4), 2322–2334.(6) Yu, Y.; Zhang, Y.; Giordano, L.; Zhu, Y. G.; Maglia, F.; Jung, R.; Gittleson, F. S.; Shao-horn, Y. Enhanced Cycling of Ni-Rich Positive Electrodes by Fluorine Modification. Journal of The Electrochemical Society 2021.