Topotactic Transformation of Lithium-Excess Spinel to Layered LiMn_0.5Ni_0.5O₂: The Interaction of 3D and 2D Li-Ion Diffusion

The cathode material plays a crucial role in shaping batteries' energy density and cycling performance, driving intensive research efforts for improvement.¹ Cathode materials such as LiNixCoyMnzO2 (NCM) and LiNixCoyAlzO2 (NCA) (Co ≤ 33%) have merged as prospective options for electric vehicle (EV) cathodes.² Despite their advantages in higher specific capacity and cost reduction when compared to cobalt-based cathodes, the high-nickel cathodes encounter challenges such as capacity degradation and instability, aggravated by the rising nickel prices. Consequently, the pursuit of novel cathode materials from economically viable and earth-abundant elements gains increasing attention, holding potential for enhanced thermal stability and cycling performance.³<br/><br/>Our recent study unveiled a novel Co-free, LxS-LiMn0.5Ni0.5O2 (LxS-LMNO) cathode based on a cubic lithium-excess spinel structure type. The LxS structure surpasses conventional spinels such as LiMn2O4 and LiMn1.5Ni0.5O4 by doubling the Li concentration in its pristine state, while maintaining its cubic symmetry. The Li/LxS-LMNO cell shows remarkable energy density, delivering ~225 mAh/g capacity from 2.5 – 5.0 V, with ~96% retention after 50 cycles. Additionally, the 3D Li-ion diffusion channels provided by its spinel framework allow faster lithium diffusion compared to layered LiMn0.5Ni0.5O2 polymorph, showing promise as a high-performance cathode.⁴<br/><br/>Our previous work demonstrated that LMNO stabilizes into the LxS structure at lower synthesis temperatures, while favoring the layered phase at higher temperatures. Such observation suggests a temperature-driven phase transition and structural integration occurring within the range typical for solid-state reactions. This dynamic interplay between these phases likely impacts the material's structure and electrochemical properties significantly. Gaining a comprehensive understanding of the temperature-dependent phase transitions holds promise for innovative cathode material design.<br/><br/>Thus, this study focuses on varying LMNO synthesis temperatures to probe the phase transition and its implications. Results indicate a parabolic relationship between the electrochemical performance of LMNO materials and synthesis temperature. This trend primarily stems from the dynamic transformation of Li-ion diffusion channels. This study not only enhance our understanding of LMNO materials but also lays the groundwork for the future development of cutting-edge cathode materials relying on the LxS structure.<br/><br/>(1) Mekonnen, Y.; Sundararajan, A.; Sarwat, A. I. A review of cathode and anode materials for lithium-ion batteries. In <i>SoutheastCon 2016</i>, 2016; IEEE: pp 1-6.<br/>(2) Choi, J. U.; Voronina, N.; Sun, Y.-K.; Myung, S.-T. Recent Progress and Perspective of Advanced High-Energy Co-Less Ni-Rich Cathodes for Li-Ion Batteries: Yesterday, Today, and Tomorrow. <i>Advanced Energy Materials </i><b>2020</b>, <i>10</i> (42), 2002027.<br/>(3) Gutierrez, A.; Tewari, D.; Chen, J.; Srinivasan, V.; Balasubramanian, M.; Croy, J. R. Earth-Abundant, Mn-Rich Cathodes for Vehicle Applications and Beyond: Overview of Critical Barriers. <i>Journal of The Electrochemical Society </i><b>2023</b>.<br/>(4) Shi, B.; Gim, J.; Li, L.; Wang, C.; Vu, A.; Croy, J. R.; Thackeray, M. M.; Lee, E. LT-LiMn 0.5 Ni 0.5 O 2: a unique co-free cathode for high energy Li-ion cells. <i>Chemical Communications </i><b>2021</b>, <i>57</i> (84), 11009-11012.