Transition metal oxide-based conversion anodes have been extensively studied as an alternative to graphite anode for lithium-ion batteries (LIBs). Recently, high-entropy oxides (HEOs) [1], a new class of materials based on single-phase multi-element solid solutions, have gathered great attention. Benefiting from the synergy among their multi-components, HEOs show great potential for application in rechargeable alkali metal-ion batteries [2]. The superior properties in Li-storage of rock salt (Mg,Co,Ni,Cu,Zn) HEO, first synthesized in 2015, have been reported by many groups [1,3,4]. Spinel-structure HEOs, first synthesized in 2018 [5], are more promising anode materials since they have higher Li-storage capacity compared to rock-salt oxides. The understanding of the charge storage mechanism in HEO-based anodes is of pivotal importance to enhance their electrochemical performance. X-ray absorption spectroscopy (XAS) studies have shed light on the lithiation mechanism in rock salt (Mg,Co,Ni,Cu,Zn) and spinel (Ti,Mn,Fe,Co,Ni) HEOs [4,6].

Anodes based on rock-salt and spinel HEOs in the form of nanofibers (NFs) show greater stability and higher reversible capacity than those prepared with HEO synthesized by conventional methods, such as sol-gel [3]. In this work, electrospun (Mn,Fe,Co,Ni,Zn) HEO NFs, produced under milder conditions than in previous studies to further reduce the environmental impact of the production process and sintering effects, are evaluated as anode active material in LIBs. An in-depth investigation of the material properties and mechanism of Li storage was carried out using a combination of analytical techniques including ex situ synchrotron XAS.

By XAS it is possible to probe the local chemical environment of atoms and their oxidation states. As it is a bulk technique, the spectra are representative for the whole sample. Mn, Fe, Ni, Co and Zn K-edges of the ex-situ electrodes were analyzed by synchrotron-based XAS in transmission mode. Investigating ex-situ samples in different states of charge, it was possible to follow the changes inside the material at each metal edge, hence shedding light on the possible mechanism of Li-storage [7], as well as the reversibility of the process. Spectral modifications were observed for all metals to different extents. For example, the changes in Co and especially in Mn spectra are more evident than those in Fe and Ni, while Zn spectra suggest that it might have a different role in the HEO structure during Li storage, compared to the other, redox metals. Also, initial cycle electrochemical data, suggesting the partial irreversibility of the original structure, were consistent with XAS results.

References

[1] C. M.Rost et al Nature Commun., 2015, 6, 8485.

[2] M. Fu et al Iscience, 2021, 24, 102177.

[3] C. Triolo et al Adv. Functi. Mater., 2022, 32, 2202892.

[4] P. Ghigna et al ACS Appl. Materials & Interfaces, 2020, 12, 50344.

[5] J. Dąbrowa et al Mater. Lett., 2018, 216, 32.

[6] T.Y. Chen et al J. Mater. Chem. A, 2020, 8, 21756.

[7] M. V. Reddy et al. Chem. Rev., 2013, 113, 5364−5457.