crystals Article Synthesis, Optical, Magnetic and Thermodynamic Properties of Rocksalt Li 1.3 Nb 0.3 Mn 0.4 O 2 Cathode Material for Li-Ion Batteries Mohamed Kamel 1 , Abanoub R. N. Hanna 2,3 , Cornelius Krellner 4 , Rüdiger Klingeler 5,6 , Mohamed Abdellah 7,8 , Mahmoud Abdel-Hafiez 1,9, * , Arafa Hassen 1, *, Ahmed S. G. Khalil 1,10 , Tarob Abdel-Baset 1,11 and Abdelwahab Hassan 1   Citation: Kamel, M.; Hanna, A.R.N.; Krellner, C.; Klingeler, R.; Abdellah, M.; Abdel-Hafiez, M.; Hassen, A.; Khalil, A.S.G.; Abdel-Baset, T.; Hassan, A. Synthesis, Optical, Magnetic and Thermodynamic Properties of Rocksalt Li 1.3 Nb 0.3 Mn 0.4 O 2 Cathode Material for Li-Ion Batteries. Crystals 2021, 11, 825. https://doi.org/10.3390/cryst 11070825 Academic Editors: Anton Meden, Jiehua Liu, Fancheng Meng and Yi Sun Received: 2 July 2021 Accepted: 14 July 2021 Published: 16 July 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 Physics Department, Faculty of Science, Fayoum University, El Fayoum 63514, Egypt; mmr06@fayoum.edu.eg (M.K.); asg05@fayoum.edu.eg (A.S.G.K.); taa03@fayoum.edu.eg (T.A.-B.); aha08@fayoum.edu.eg (A.H.) 2 Institut für Festkörperforschung, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany; abanoub.hanna@helmholtz-Berlin.de 3 Helmholtz-Zentrum Berlin für Materialien und Energie, Hahn-Meitner Platz 1, 14109 Berlin, Germany 4 Physikalisches Institut, Goethe-University Frankfurt, 60438 Frankfurt am Main, Germany; krellner@physik.uni-frankfurt.de 5 Kirchhoff Institute of Physics, Heidelberg University, 69120 Heidelberg, Germany 6 Centre for Advanced Materials (CAM), Heidelberg University, 69120 Heidelberg, Germany; ruediger.klingeler@kip.uni-heidelberg.de 7 Ångström Laboratory, Department of Chemistry, Uppsala University, Box 523, 75120 Uppsala, Sweden; Mohamed.Qenawy@kemi.uu.se 8 Department of Chemistry, Qena Faculty of Science, South Valley University, Qena 83523, Egypt 9 Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden 10 Materials Science and Engineering Department, School of Innovative Design Engineering, Egypt-Japan University of Science and Technology (E-JUST), P.O. Box 179, New Borg El-Arab City 21934, Egypt 11 Department of Physics, Faculty of Science, Taibah University, Yanbu 46423, Saudi Arabia * Correspondence: mahmoud.hafiez@physics.uu.se (M.A.-H.); ash02@fayoum.edu.eg (A.H.); Tel.: +46-72-831-37-11 (M.A.-H.) Abstract: Since the discovery of the reversible intercalation of lithium-ion materials associated with promising electrochemical properties, lithium-containing materials have attracted attention in the research and development of effective cathode materials for lithium-ion batteries. Despite various studies on synthesis, and electrochemical properties of lithium-based materials, fairly little fundamen- tal optical and thermodynamic studies are available in the literature. Here, we report on the structure, optical, magnetic, and thermodynamic properties of Li-excess disordered rocksalt, Li 1.3 Nb 0.3 Mn 0.4 O 2 (LNMO) which was comprehensively studied using powder X-ray diffraction, transient absorption spectroscopy, magnetic susceptibility, and low-temperature heat capacity measurements. Charge carrier dynamics and electron–phonon coupling in LNMO were studied using ultra-fast laser spec- troscopy. Magnetic susceptibility and specific heat data are consistent with the onset of long-range antiferromagnetic order at the Néel temperatures of 6.5 (1.5) K. The effective magnetic moment of LNMO is found to be 3.60 μ B . The temperature dependence of the inverse magnetic susceptibility follows the Curie–Weiss law in the high-temperature region and shows negative values of the Weiss temperature 52 K (3), confirming the strong AFM interactions. Keywords: Li-ion batteries; Li 1.3 Nb 0.3 Mn 0.4 O 2 ; magnetic susceptibility; specific heat 1. Introduction Lithium-ion batteries (LIBs) are one of the most auspicious energy storage technolo- gies for smartphones, laptops, electric hybrid vehicles, and renewable energy systems [1–3]. LIBs consist of two electrodes called the anode (negative) and the cathode (positive) sepa- rated by an electrolyte that can be a liquid or a solid [4,5]. Various alternative anode and Crystals 2021, 11, 825. https://doi.org/10.3390/cryst11070825 https://www.mdpi.com/journal/crystals