Yong-Seok Choi1,2,Stephanie Linnell3,2,A. Robert Armstrong3,2,David Scanlon1,2
University College London1,The Faraday Institution2,University of St Andrews3
Yong-Seok Choi1,2,Stephanie Linnell3,2,A. Robert Armstrong3,2,David Scanlon1,2
University College London1,The Faraday Institution2,University of St Andrews3
Having dominated the market for over the past three decades, Li-ion batteries (LIBs) are now faced with a challenge for the use in grid-scale energy storages because of the rarity of Li resources. To meet the growing demands of power sources, Na-ion batteries (NIBs) have emerged as a promising alternative for LIBs owing to the high abundance of sodium. Despite the considerable attention to NIBs, their overall electrochemical performance remains inferior to LIBs because of the less negative redox potential and larger ionic radius of Na <sup>[1,2]</sup>. Thus, the development of Na-ion cathodes with high energy densities and long cycle lives are critical for NIBs to be widely applied as an alternative to LIBs.<br/>Among various crystalline phases reported to date, P3-type cathodes can be synthesized at lower temperatures <sup>[3]</sup> and have a higher Na content <sup>[4]</sup> than P2-type oxides. Moreover, thanks to the prismatic Na sites, P3-type cathodes have larger interlayer space than O3-type counterpart <sup>[3]</sup>, endowing improved rate capability. To maximize the performance of P3-type cathodes, extensive studies have been conducted on doping other elements on the transition metal (<i>TM</i>) sites. <sup>[5]</sup> In the journey of research on heteroatom doping, various relationships between dopant property and cathode performance have been suggested: 1) dopant possessing higher electronegativity than <i>TM</i> elements increases thermodynamic redox potentials, according to the inductive effect <sup>[6]</sup>. 2) Dopants with lower oxidations states and smaller ionic radii (e.g., Mg<sup>2+</sup>, Al<sup>3+</sup>) widens the interlayer distances while causing <i>TM</i>O<sub>6</sub> octahedra shrinkage, enhancing the rate performance <sup>[7,8]</sup>. 3) The redox inactivity of dopants stabilizes the <i>TM</i> layer upon battery cycling, which mitigates the capacity fading <sup>[9]</sup>. However, the selection of a single dopant satisfying all above criteria is nearly impossible, as electronegativity is positively correlated with oxidation states and ionic radii. From this perspective, dual or multiple doping is inevitable to design future cathodes with high performance.<br/>In this study, using combined technique of experiments and first-principles calculations, we investigated the effect of two dopants (Ti and Zn) with contrasting properties (electronegativity, oxidation state, and ionic radii) on P3-type Na<sub>0.7</sub>Mn<sub>0.75</sub>Ni<sub>0.25</sub>O<sub>2</sub>, aiming for the synergetic effect of dual doping. Galvanostatic cycling test showed that each dopant has different effect on cyclability; Zn and Ti dopants improved the cyclic stability at the early and later stage of cycling, respectively. Furthermore, the redox voltages of Na<sub>0.7</sub>Mn<sub>0.75</sub>Ni<sub>0.25</sub>O<sub>2</sub> decrease after doping, where the degree of decrement is greater for Zn dopant. When both elements are doped, the resulting cathode exhibits greater performance than Zn- or Ti-doped samples in both cyclability and rate performance. Considering the recent interpretations on dopant-property relationships, we interpret the observed behaviours employing first-principles calculations. Finally, based on the conclusions obtained from each dopant effect, we discuss how the combination of dopants with contrasting properties can lead to the improved Na-ion cathodes.<br/><br/><b>References</b><br/>[1] N.Tapia-Ruiz et al., J. Phys. Energy 3.3 (2021): 031503<br/>[2] Y.U. Park, et al., J. Amer. Chem. Soc. 135.37 (2013): 13870-13878<br/>[3] L.Xian, et al., J. Alloys Compd. 905 (2022): 163965<br/>[4] Y.Wang, et al., Chem. Eng. J. 360 (2019): 139-147<br/>[5] Y.Li, et al., Adv. Energy Mater. 10.27 (2020): 2000927<br/>[6] D.A.Kuznetsov, et al., Joule 2.2 (2018): 225-244<br/>[7] N.Tapia-Ruiz et al., Energy Environ. Sci. 11.6 (2018): 1470-1479<br/>[8] C. Soares, B. Silvan, <b><i><u>Y.S. Choi</u></i></b>, V. Celorrio, V.R. Seymour, G. Cibin, J.M. Griffin, D.O. Scanlon and N. Tapia-Ruiz, J. Mater. Chem. A 10.13 (2022): 7341-7356<br/>[9] S.F. Linnell, E.J. Kim, <b><i><u>Y.S. Choi</u></i></b>, M. Hirsbrunner, S. Imada, A. Pramanik, A.F. Cuesta, D. Miller, E. Fusco, B.E. Bode, J.T.S. Irvine, L.C. Duda, D.O. Scanlon, A.R. Armstrong, J. Mater. Chem. A 10.18 (2022): 9941-9953