Segregation Phenomena in Rock-Salt High Entropy Oxides for Lithium Battery Applications Studied by In Situ Annealing in a Scanning Transmission Electron Microscope

Recently high-entropy oxides were proposed as promising materials for lithium battery anodes, with reports showing improvements in the storage capacity and cycling ability of the rock-salt type materials attributed to entropic stabilization due to random distribution of metal cations within the lattice (see e.g. [1]). However, multiple further studies suggested that these materials are not behaving as a homogenous mix of cations: in situ XAS investigation showed that lithiation-delithiation follows a multistep path, with some irreversible processes taking place [2], and recent localized spectroscopy results showed nanoscale segregation and differential movement of the constituent cations during charging and discharging [3]. It is therefore very important to evaluate the compositional stability of these materials at different elevated temperatures, given that charge/discharge processes can be highly exothermic.<br/>In this study we are exploring the elemental distribution in the equimolar rock-salt high-entropy oxide of Li, Mg, Mn, Co, Ni, Cu and Zn using compositional mapping with Electron Energy Loss spectroscopy in a Scanning Transmission Electron Microscope using <i>in situ</i> Protochips Aduro platform to heat the powders to temperatures ranging from 500°C to 950°C. The initial powders are either the mixture of initial oxides or pre-annealed <i>ex situ</i>, ball-milled before the <i>in situ</i> heating experiment.<br/>Our results suggest that, even with a highly homogenized precursors, the most uniform distribution of cations is only achieved at high annealing temperatures, such as 950°C, and thermal treatment much below that temperature results in swift segregation of primarily Cu into distinct agglomerates. It takes as little as 1.5 hours at 500°C for the powder pre-annealed at 950°C to show Cu segregation. We also find that the reducing vacuum atmosphere inside the microscope contributes to the driving force for the segregation; while the powders annealed at 950°C inside the microscope from the precursors are the most homogeneous, they still show some segregation compared to powders annealed <i>ex situ</i> at the same temperature which do not.<br/>These results confirm earlier findings about differential behavior of cations in these materials as they undergo thermal treatment and chemical transformations and suggest that controlling heat distribution within these anodes during charging and discharging can be very important for cycling stability [4].<br/>References<br/>[1] Sarkar, A., Velasco, L., Wang, D. <i>et al.</i> High entropy oxides for reversible energy storage. <i>Nat Commun</i> <b>9</b>, 3400 (2018). DOI:10.1038/s41467-018-05774-5<br/>[2] P. Ghigna, L. Airoldi, M. Fracchia <i>et al.</i> Lithiation Mechanism in High-Entropy Oxides as Anode Materials for Li-Ion Batteries: An Operando XAS Study, <i>ACS Applied Materials & Interfaces</i> <b>12, </b>50344 (2020). DOI: 10.1021/acsami.0c13161<br/>[3] Wang, K., Hua, W., Huang, X. <i>et al.</i> Synergy of cations in high entropy oxide lithium ion battery anode. <i>Nat Commun</i> <b>14</b>, 1487 (2023). DOI:10.1038/s41467-023-37034-6<br/>[4] This work was supported by the DOE office of Science, Basic Energy Sciences, Materials Science and Engineering Division.