Oxygen Redox Activity in P2 Layered Oxides as High Energy Cathodes in Na-Ion Batteries—New Design Hints from First-Principles

Na-ion batteries (NIBs) are rapidly emerging as promising post-Lithium technology for large-scale applications, thanks to the wide availability and low cost of raw materials [1]. Design and optimization of highly efficient active materials are major issues for their effective deployment [2]. Layered transition metal oxides (NaxTMO2) have shown outstanding performances as high-energy cathode materials in NIB cells and exhibited the chance to attain larger specific capacity by enabling anionic reactions at high operating voltage [3, 4]. This represents a new paradigm in the development of positive electrodes, but the O2-/O2n-/O2 redox processes need to be finely controlled to prevent the release of molecular oxygen and thus huge capacity loss. We report a first-principles investigation of P2-type Mn-defective layered oxides with different metal doping at the TM site by means of PBE+U-D3(BJ) calculations. Structural and electronic features are dissected for each redox-active element in NaxTM0.25Mn0.68O2 (TM = Ni, Fe) materials as function of sodiation degree corresponding to different states of charge. We address the oxygen redox activity by considering the formation of oxygen vacancies and dioxygen metal complexes at low Na loads (i.e., high voltage range). Low-energy superoxide moieties with different coordination geometries are predicted to be formed at x Na = 0.25 in Mn-deficient sites, while the x Na = 0.125 content enables the release of molecular O2 via preferential breaking of Ni-O bonds. Mechanistic insights show that dioxygen formation is driven by the TM-O covalency and unveil that O2 loss can be effectively suppressed by Fe and Ru doping. Our findings pave the route for the rational design of high-energy NaxTMO2 cathodes that feature enhanced reversible capacity and thus boost the development of efficient NIB devices. These outcomes are subject of our recent publications on ACS Energy Letters and Journal of American Ceramic Society [5, 6].<br/>References:<br/>[1] B. Dunn, et al., Science, 334(6058), 928-935 (2011)<br/>[2] Y. Huang, et al., ACS Energy Lett., 3(7), 1604-1612 (2018)<br/>[3] Q. Wang, et al., Nat. Mater., 20(3), 353-361 (2021)<br/>[4] M. Ben Yahia, et al., Nat. Mater., 18(5), 496-502 (2019).<br/>[5] A. Massaro, et al. ACS Energy Lett., 6, 2470-2480 (2021).<br/>[6] A. Massaro, et al. Journal of the American Ceramic Society, in press (2022),