Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej In situ synchrotron radiation diffraction study of the Li + de/intercalation behavior in spinel LiNi 0.5 Mn 1.5 O 4-δ Suning Wang a,b , Weibo Hua b,c, ⁎ , Shuo Zhou a , Xiafeng He a , Laijun Liu a, ⁎ a Guangxi Key Laboratory of Optical and Electronic Materials and Devices, College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, China b Institute for Applied Materials (IAM), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany c State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China HIGHLIGHTS • Oxygen content control of LiNi 0.5 Mn 1.5 O 4 was achieved via a microwave annealing treatment; • The prepared non-stoichiometric LiNi 0.5 Mn 1.5 O 4-δ material delivers excellent rate capability; • A solid-solution-like reaction in the prepared LiNi 0.5 Mn 1.5 O 4-δ during cycling was deciphered. ARTICLE INFO Keywords: Spinel LiNi 0.5 Mn 1.5 O 4 Cathode material Rate capability Oxygen vacancies Structural evolution In situ synchrotron radiation diffraction ABSTRACT The oxygen vacancies are usually beneficial to Li-ion transport in high-voltage spinel LiNi 0.5 Mn 1.5 O 4 (LNMO) cathode material, but the influence of the oxygen deficiencies in the disordered spinel LNMO oxide on phase transition mechanism during charging/discharging remains unknown. Herein, the oxygen stoichiometry control of LNMO was achieved through a microwave annealing treatment. The prepared non-stoichiometric LiNi 0.5 Mn 1.5 O 4-δ (Fd − 3m) materials delivered a pronounced improvement of rate capability, e.g. the discharge capacity at an ultra-fast rate (10 C) is up to 104.3 mA h g -1 . Instead of the typical two-step phase transformation process, a solid-solution-like reaction in the as-synthesized LiNi 0.5 Mn 1.5 O 4-δ (Fd − 3m) during cycling was deci- phered by an in situ synchrotron radiation diffraction analysis. These findings shed light on the phase transition mechanism of defective spinel oxides with an enhanced electrochemical properties. 1. Introduction Spinel LiNi 0.5 Mn 1.5 O 4 (LNMO) cathode material can exhibit around 20% excess energy density (650 Wh kg -1 ) compared with today’s tra- ditionally used cathodes such as LiCoO 2 , LiNi 1/3 Co 1/3 Mn 1/3 O 2 and LiFePO 4 [1–3]. Its high working potential at about 4.7 V makes it a promising candidate cathode material for 5 V lithium-ion batteries (LIBs) in vehicle applications [4,5]. In addition, LNMO has a fast in- trinsic Li-ion diffusion within the three dimensional (3D) spinel struc- ture, leading to its superior rate capability and cycleability [6,7]. Two types of crystallographic structure are generally tied to LNMO, i.e, the ordered LiNi 0.5 Mn 1.5 O 4 (space group P4 3 32) and the disordered non- stoichiometric LiNi 0.5 Mn 1.5 O 4-δ (Fd − 3m) [8]. The Li ions and transition metal (TM, TM = Ni and Mn) cations occupy the tetrahedral (8a site) and octahedral (16d site) positions, respectively, in the disordered non- stoichiometric LNMO; while the Ni 2+ and Mn 4+ ions locate at two different octahedral sites (i.e. 4a and 16d) in the ordered stoichiometric LNMO [9]. Most of the prepared LNMO are disordered non-stoichiometric LNMO due to the generation of oxygen vacancies during high-tem- perature solid-solution reaction [10,11]. Simultaneously, the oxygen loss during synthesis of LNMO causes a reduction of Mn 4+ to Mn 3+ due to remaining the charge neutrality. As a result, a larger ionic radius of Mn 3+ compared to Mn 4+ creates a bigger unit-cell volume of cubic spinel LNMO, which facilities the rapid Li-ion transport in the LNMO cathode. Previous studies have been suggested that the Mn 3+ is elec- trochemically active and contributes to a small plateau at approxi- mately 4.0 V [12–14]. On the other hand, Mn 3+ (electron configuration 3d 4 ) makes LNMO suffer a Jahn-Teller distortion and a slow dissolution of manganese ions into electrolyte through a disproportionation reac- tion (2Mn 3+ → Mn 2+ + Mn 4+ ). This results in a serious capacity https://doi.org/10.1016/j.cej.2020.125998 Received 7 March 2020; Received in revised form 18 May 2020; Accepted 18 June 2020 ⁎ Corresponding authors. E-mail addresses: weibo.hua@kit.edu (W. Hua), ljliu2@163.com (L. Liu). Chemical Engineering Journal 400 (2020) 125998 1385-8947/ © 2020 Elsevier B.V. All rights reserved. T