Synthesis, Structure, Electrochemical Mechanisms, and Atmospheric Stability of Mn-Based Layered Oxide Cathodes for Sodium Ion Batteries

ConspectusThe commercialization of lithium ion batteries (LIBs) triggered a new era of portable electronics and electric vehicles, which changed our daily life remarkably. Meanwhile, LIBs are promising as large-scale storage batteries in green electric grids by using renewable energy as the primary energy source. Driven by the concerns of lithium depletion and the turbulent price of Li, Ni, and Co mineral resources, sodium ion batteries (NIBs) have been intensively investigated and are becoming a strong competitor for large-scale energy storage, such as in national grids. As an inevitable component of NIBs, the cathode determines all of the critical properties of NIBs, such as cost, safety, energy/power densities, and cycling life, and an ideal cathode material should be sustainable and easy-to-use in scalable production, transportation, and storage. Layered sodium transition metal oxide (NaxTMO2), especially the Mn-based NaxTMO2, has been chosen as a family of leading cathodes for the upcoming commercial NIBs because of a great variety of compositions and redox-active elements, such as Mn, Fe, Cu, Ni, Co, Cr, Ir, Ru, and even O. The success or failure of the commercialization of these materials relies on the resolution of two critical challenges. The first is the electrochemical irreversibility related to the phase transformations and electrolyte decomposition during repeated charging and discharging processes. The second challenge is the chemical and structural sensitivity when Mn-based NaxTMO2 is exposed to moist air. These air-sensitive oxides need to be prepared and stored in inert or dry atmospheres, thus increasing the costs of production, storage, and transportation. During the past decade, we focused on investigating the degradation mechanisms and exploring effective strategies to enhance the atmospheric stability and electrochemical reversibility of Mn-based NaxTMO2. In addition, the materials synthesis and charge-compensation mechanisms, which are critical to the electrochemical characteristics of sodium layered oxides, have also been taken into consideration in our research.In this Account, we highlight recent efforts to understand the synthesis, phase diversity, atmospheric stability, and electrochemical reaction mechanisms of NaxMnO2 and their analogues, with a special focus on the structure and its evolutions. This Account starts with introducing the polymorphism and synthesis of the NaxMnO2 series. Next, we present the cause and control of off-stoichiometry in the P2-type NaxMnO2 series as well as its effect on electrochemical performance and structural transformations during Na+ (de)intercalation. Then we summarize the mechanical, electrochemical, and atmospheric structural stability as well as the corresponding modification strategies of Mn-based NaxTMO2. At the end of this Account, we emphasize the charge-compensation mechanisms with a special focus on anionic redox reactions. On the basis of these insights, a brief outlook and discussion of the future development of Mn-based NaxTMO2 with high performance are provided. This Account will provide significant understanding and inspiration toward cost-effective and high-performance NaxTMO2.