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energies Article Exploring the Charge Compensation Mechanism of P2-Type Na0.6Mg0.3Mn0.7O2 Cathode Materials for Advanced Sodium-Ion Batteries Chen Cheng 1, Manling Ding 1, Tianran Yan 1, Kehua Dai 2, Jing Mao 3, Nian Zhang 4, Liang Zhang 1,* and Jinghua Guo 5,6,* 1 2 3 4 5 6 Received: 2 September 2020; Accepted: 29 October 2020; Published: 2 November 2020 Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University, 199 Ren’ai Road, Suzhou 215123, China; 20184214128@stu.suda.edu.cn (C.C.); 20184214029@stu.suda.edu.cn (M.D.); 20194214102@stu.suda.edu.cn (T.Y.) College of Chemistry, Tianjin Normal University, Tianjin 300387, China; Daikh@smm.neu.edu.cn School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China; maojing@zzu.edu.cn State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, China; zhangn@mail.sim.ac.cn Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Department of Chemistry and Biochemistry, University of California, Santa Cruz, CA 95064, USA * Correspondence: liangzhang2019@suda.edu.cn (L.Z.); jguo@lbl.gov (J.G.) Abstract: P2-type sodium layered transition metal oxides have been intensively investigated as promising cathode materials for sodium-ion batteries (SIBs) by virtue of their high specific capacity and high operating voltage. However, they suffer from problems of voltage decay, capacity fading, and structural deterioration, which hinder their practical application. Therefore, a mechanistic understanding of the cationic/anionic redox activity and capacity fading is indispensable for the further improvement of electrochemical performance. Here, a prototype cathode material of P2-type Na0.6Mg0.3Mn0.7O2 is comprehensively investigated, which presents both cationic and anionic redox behaviors during the cycling process. By a combination of soft X-ray absorption spectroscopy and electroanalytical methods, we unambiguously reveal that only oxygen redox reaction is involved in the initial charge process, then both oxygen and manganese participate in the charge compensation in the following discharge process. In addition, a gradient distribution of Mn valence state from surface to bulk is disclosed, which could be mainly related to the irreversible oxygen activity during the charge process. Furthermore, we find that the average oxidation state of Mn is reduced upon extended cycles, leading to the noticeable capacity fading. Our results provide deeper insights into the intrinsic cationic/anionic redox mechanism of P2-type materials, which is vital for the rational design and optimization of advanced cathode materials for SIBs. Keywords: sodium-ion batteries; P2-tpye oxides; charge compensation mechanism; X-ray absorption spectroscopy; electronic structure 1. Introduction The rapid development of electronic devices and electric vehicles calls for electrochemical energy storage with an ever-increasing energy density. At the moment, lithium-ion batteries (LIBs) are dominating the market because of their high operating voltages and superior energy densities [1]. However, lithium availability exhaustion and rising costs have driven researchers to explore suitable Energies 2020, 13, 5729; doi:10.3390/en13215729 www.mdpi.com/journal/energiesPDF Image | Cathode Materials for Advanced Sodium-Ion Batteries
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