materials Article New O3-Type Layer-Structured Na 0.80 [Fe 0.40 Co 0.40 Ti 0.20 ]O 2 Cathode Material for Rechargeable Sodium-Ion Batteries Daniel A. Anang 1,2 , Deu S. Bhange 3 , Basit Ali 1 and Kyung-Wan Nam 1, *   Citation: Anang, D.A.; Bhange, D.S.; Ali, B.; Nam, K.-W. New O3-Type Layer-Structured Na 0.80 [Fe 0.40 Co 0.40 Ti 0.20 ]O 2 Cathode Material for Rechargeable Sodium-Ion Batteries. Materials 2021, 14, 2363. https:// doi.org/10.3390/ma14092363 Academic Editor: Dimitra Vernardou Received: 2 April 2021 Accepted: 29 April 2021 Published: 1 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 04620, Korea; taadjah@yahoo.com (D.A.A.); basitalikhan077@gmail.com (B.A.) 2 Department of Chemical Engineering, Kwame Nkrumah University of Science and Technology, PMB, Kumasi, Ghana 3 Department of Chemistry, Shivaji University, Kolhapur 416004, India; bhangeds@yahoo.co.in * Correspondence: knam@dongguk.edu; Tel.: +82-2-2290-4978; Fax: +82-2-2268-8550 Abstract: Herein, we formulated a new O3-type layered Na 0.80 [Fe 0.40 Co 0.40 Ti 0.20 ]O 2 (NFCTO) cath- ode material for sodium-ion batteries (SIBs) using a double-substitution concept of Co in the parent NaFe 0.5 Co 0.5 O 2 , having the general formula Na 1-x [Fe 0.5–x/2 Co 0.5–x/2 M 4+ x ]O 2 (M 4+ = tetravalent ions). The NFCTO electrode delivers a first discharge capacity of 108 mAhg −1 with 80% discharge capacity retention after 50 cycles. Notably, the first charge–discharge profile shows asymmetric yet reversible redox reactions. Such asymmetric redox reactions and electrochemical properties of the NFCTO electrode were correlated with the phase transition behavior and charge compen- sation reaction using synchrotron-based in situ XRD and ex situ X-ray absorption spectroscopy. This study provides an exciting opportunity to explore the interplay between the rich chemistry of Na 1–x [Fe 0.5–x/2 Co 0.5–x/2 M 4+ x ]O 2 and sodium storage properties, which may lead to the development of new cathode materials for SIBs. Keywords: Na-ion battery; layered structure; cathode; in situ XRD; XANES 1. Introduction Thanks to their high energy and power density and long cycle life, lithium-ion batteries (LIBs) are currently the most marketable rechargeable batteries for mobile electronics applications [1]. During the past decade, LIBs have been enormously successful in the transportation industry, as they are perceived as a better option in mitigating environmental pollution created by vehicles using conventional combustion engines [2]. Despite this success, the uneven distribution and nonabundant nature of Li resources in the earth crust make LIB technology unsustainable in large-scale energy storage systems (ESSs) in the long term [3,4]. Sodium-ion batteries (SIBs) offer an attractive alternative to LIBs in such large-scale ESSs thanks to their similar intercalation chemistry to LIBs, low cost, and high material abundance [1]. Recently, various sodium (Na) layered oxide materials have been discovered, and their electrochemical properties have been critically examined [4–6]. In particular, the α-NaFeO 2 material reported by Yabuuchi et al. exhibited a flat voltage plateau at 3.3 V vs. Na + /Na with 80 mAhg −1 reversible capacity coupled with reasonable stability up to 30 cycles [7]. However, when the operating voltage was increased above 3.4 V, significant capacity fading was observed. Structural studies revealed Fe ion migration from the transition metal (TM) layer to the Na layer, leading to irreversible structural change during cycling at higher voltages. Subsequently, Yoshida et al. designed NaFe 0.5 Co 0.5 O 2 material by substituting half of Fe by Co and obtained improved electrochemical performance over NaFeO 2 [8]. This cathode material extended the discharge capacity to 160 mAhg −1 with ~85 % discharge capacity retention after five cycles and showed high rate (30C) capability. Despite these performances, it was concluded that the Co content must be Materials 2021, 14, 2363. https://doi.org/10.3390/ma14092363 https://www.mdpi.com/journal/materials