Citation: Lin, Y.; Välikangas, J.; Sliz, R.; Molaiyan, P.; Hu, T.; Lassi, U. Optimized Morphology and Tuning the Mn 3+ Content of LiNi 0.5 Mn 1.5 O 4 Cathode Material for Li-Ion Batteries. Materials 2023, 16, 3116. https:// doi.org/10.3390/ma16083116 Academic Editor: Alessandro Dell’Era Received: 17 March 2023 Revised: 5 April 2023 Accepted: 11 April 2023 Published: 15 April 2023 Copyright: © 2023 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/). materials Article Optimized Morphology and Tuning the Mn 3+ Content of LiNi 0.5 Mn 1.5 O 4 Cathode Material for Li-Ion Batteries Yan Lin 1, * , Juho Välikangas 1,2 , Rafal Sliz 3 , Palanivel Molaiyan 1 , Tao Hu 1 and Ulla Lassi 1,2, * 1 Research Unitof Sustainable Chemistry, Faculty of Technology, University of Oulu, 90570 Oulu, Finland 2 Kokkola University Consortium Chydenius, University of Jyvaskyla, 67100 Kokkola, Finland 3 Optoelectronics and Measurement Techniques Unit, University of Oulu, 90570 Oulu, Finland * Correspondence: yan.lin@oulu.fi (Y.L.); ulla.lassi@oulu.fi (U.L.) Abstract: The advantages of cobalt-free, high specific capacity, high operating voltage, low cost, and environmental friendliness of spinel LiNi 0.5 Mn 1.5 O 4 (LNMO) material make it one of the most promising cathode materials for next-generation lithium-ion batteries. The disproportionation reac- tion of Mn 3+ leads to Jahn–Teller distortion, which is the key issue in reducing the crystal structure stability and limiting the electrochemical stability of the material. In this work, single-crystal LNMO was synthesized successfully by the sol-gel method. The morphology and the Mn 3+ content of the as-prepared LNMO were tuned by altering the synthesis temperature. The results demonstrated that the LNMO_110 material exhibited the most uniform particle distribution as well as the presence of the lowest concentration of Mn 3+ , which was beneficial to ion diffusion and electronic conductivity. As a result, this LNMO cathode material had an optimized electrochemical rate performance of 105.6 mAh g −1 at 1 C and cycling stability of 116.8 mAh g −1 at 0.1 C after 100 cycles. Keywords: LiNi 0.5 Mn 1.5 O 4 ; sol-gel method; Mn 3+ content; cathode materials; li-ion battery 1. Introduction The development of the battery manufacturing ecosystem is crucial to the ambitious worldwide push toward renewable energy resources and electric vehicles (EVs). High energy density lithium-ion batteries (LIBs) are urgently needed to meet the soaring de- mand for diverse portable gadgets, hybrid electronic devices (HEVs), and electric vehicles (EVs) [1–4]. Since cathode materials are responsible for a significant portion of the weight and expense in state-of-the-art LIBs, the development of low-cost and high-performance cathode is a considerable research direction for next-generation LIBs [5–9]. Currently, commercial cathodes, such as those made of lithium cobalt oxide (LiCoO 2 ) and its deriva- tives LiNi x Mn y Co 1−x−y O 2 (NMC) and LiNi x Co y Al 1−x−y O 2 (NCA), are commonly utilized, although the cost of metallic cobalt materials (70,000 USD per tonne) has increased dra- matically [10–13]. Therefore, many kinds of cobalt-free materials, including LiFePO 4 (LFP), LiNiO 2 (LNO), LiMn 2 O 4 (LMO), and LiNi 0.5 Mn 1.5 O 4 (LNMO), have also been investigated to produce cathode materials that are abundant and affordable [14–17]. As is known to all, the energy density of LIBs depends not only on the specific capacity but also on the operating voltage. To achieve high energy density LIBs, it is effective to create high-voltage cathode materials. Spinel LNMO stands out for its high operating voltage platform (4.7 V vs. Li/Li + ) and high energy density (~650 Wh kg −1 ). However, the disordered LNMO suffers from the intrinsic defect of the presence of Mn 3+ and oxygen vacancies due to the high- temperature calcination. Due to the disproportionation reactions (2Mn 3+ → Mn 2+ + Mn 4+ ), the presence of Mn 3+ leads to transition metal dissolution and Jahn-Teller distortion, result- ing in structural instability and capacity decay [18–20]. The most popular synthesis techniques for LNMO are the sol-gel, co-precipitation, and solid-state reaction approaches [21–23]. Among them, the sol-gel method is frequently employed due to its advantages of product uniformity, low cost, and easy operation. Materials 2023, 16, 3116. https://doi.org/10.3390/ma16083116 https://www.mdpi.com/journal/materials