Unravelling the Effect of Sodium Doping on Li₂MnO₃nanostructured Cathode Materials

Abstract

The quest to develop affordable, high-performance electrode materials for lithium-ion batteries in electric vehicles is driving scientific innovation forward. Li 2 MnO 3 , as a prospective high-capacity (459 mAh.g -1 ) cathode, suffers from capacity degradation and voltage decay during the cycling process. Nanostructuring and incorporating sodium ions into lithium sites can mitigate voltage decay by limiting transition metal migration, impeding oxygen loss, and improving lithium diffusion by lowering the activation energy of Li-rich layered host materials. Herein, sodium-incorporated nonospherical structures of the form Li 2-x Na x MnO 3 (0 ≤ x ≤ 2), generated through the amorphisation and recrystallisation (A+R) technique show remarkable structural stability and enhanced Li-ion mobility owing to the enlarged lithium lattice upon sodium entrance. Specifically, the Li 1.95 Na 0.05 NaO 3 nanosphere, displays more stable Li + /Mn 4+ layers which will greatly contribute to its capacity. The microstructures associated with this configuration show well-ordered layers with high lithium kinetics and lower activation energy compared to pristine Li 2 MnO 3 . Characterisation of the x-ray diffraction (XRD) patterns revealed peak broadening along with the shifting of peaks at 2Θ~38 to the right due to the enlarged lithium layers occupied by sodium ions to facilitate lithium diffusion. Moreover, the undesired phase transformation from layered to spinel was observed at a later stage of charge for the sodium-doped systems, suggesting that the presence of sodium stabilizes the structure and minimizes the migration of manganese into lithium layers. These findings may lead to solutions for problems such as structural instability, lattice oxygen loss, and capacity decay in layered lithium oxide materials.