Structural Evolution of Air-Exposed Layered Oxide Cathodes for Sodium-Ion Batteries: An Example of Ni-doped Na_xMnO₂

Abstract

Sodium-ion batteries have recently aroused the interest of industries as possible replacements for lithium-ion batteries in some areas. With their high theoretical capacities and competitive prices, P2-type layered oxides (Na x TMO 2 ) are among the obvious choices in terms of cathode materials. On the other hand, many of these materials are unstable in air due to their reactivity toward water and carbon dioxide. Here, Na 0.67 Mn 0.9 Ni 0.1 O 2 (NMNO), one of such materials, has been synthesized by a classic sol−gel method and then exposed to air for several weeks as a way to allow a simple and reproducible transition toward a Na-rich birnessite phase. The transition between the anhydrous P2 to the hydrated birnessite structure has been followed via periodic XRD analyses, as well as neutron diffraction ones. Extensive electrochemical characterizations of both pristine NMNO and the air-exposed one vs sodium in organic medium showed comparable performances, with capacities fading from 140 to 60 mAh g −1 in around 100 cycles. Structural evolution of the air-exposed NMNO has been investigated both with ex situ synchrotron XRD and Raman. Finally, DFT analyses showed similar charge compensation mechanisms between P2 and birnessite phases, providing a reason for the similarities between the electrochemical properties of both materials.

First-Principles Insights on Solid-State Phase Transitions in P2-NaxMnO2-Based High Energy Cathode during Na-Ion Battery Operations

“…Lithium-ion batteries (LIBs) have revolutionized the field of portable electronics and electric vehicles thanks to their high energy density, long cycle life, and negligible self-discharge rates. − Nevertheless, the dependence on rare and expensive raw materials, such as cobalt and lithium itself, has raised concerns about resource availability and environmental implications. − Researchers are actively exploring alternative battery technologies that can ensure sustainability, improved safety, and high energy storage capacity required for large-scale applications. − Among the post-lithium battery options, Na-ion batteries (NIBs) . ,− Benefiting from the abundance of sodium resources and sharing similar intercalation chemistry with state-of-the-art LIBs, ,, NIBs offer potential advantages, particularly for large-scale energy storage, where cost-effectiveness and durability are crucial parameters. , For this purpose, designing new cathode materials that comprise earth-abundant, nontoxic, and highly stable components is of paramount importance, as the positive electrode significantly impacts the battery performance and accounts for one-third of the total battery cost. − Layered sodium transition metal oxides (Na x TMO 2 ) based on abundant metals have attracted considerable attention, owing to their relatively low manufacturing cost, while also exhibiting large specific capacity and high-rate capabilities. ,,,− The high abundance, affordability, and low toxicity of manganese make Mn-rich oxides the most viable choice. , Various polymorphs of NaMnO 2 (NMO) have been considered as ideal NIB cathodes, with appropriate doping at the TM site allowing an easy tuning of electrochemical performances − and even enabling additional redox activity relying on anion-based processes. − Nonetheless, the practical application of NMO cathodes is usually limited by detrimental phase transitions that can occur upon charge/discharge. ,,,− The subsequent extraction and reinsertion of Na + ions can drive the evolution between phases featuring different shapes and volumes, preventing good reversibility an...…”

P2-Type Na0.84Li0.1Ni0.27Mn0.63O2-Layered Oxide Na-Ion Battery Cathode: Structural Insights and Electrochemical Compatibility with Room-Temperature Ionic Liquids

“… 6 Several key aspects still characterize this family of materials, including irreversible phase transformations during cycling, poor air stability, and complex charge-compensation mechanisms. 7 Fundamental understanding of degradation behavior upon cycling and air exposure, 8 correlating structure-performance relationships, and establishing general design strategies must be considered for their effective exploitation in commercial NIBs. 9 Recent progress in this field has highlighted critical factors that would balance the overall material efficiency.…”