Designing Dopant-Stabilized Layered Oxides for Aqueous Na-Ion Batteries: The Effect of Li–Ti Doping, Redox Chemistry, and Structural Dynamics

Sodium manganese oxide (Na x MnO 2 ) is the preferred cathode for aqueous sodium-ion batteries (ASIBs) in large-scale energy storage systems. However, the structural instability, presence of the Jahn–Teller-active element, manganese dissolution, and surface degradation during cycling lead to capacity fading and unstable interface formation when used as a cathode in ASIBs. Additionally, the lattice changes during charge–discharge results in nanovoid and crack formation. Addressing the issue of the construction of high-capacity cathode materials with superior stability is most important to improving the performance of ASIBs. Here, these significant issues are addressed by incorporating heteroatoms in the structure; a Na 0.7 Fe 0.1 Cu 0.1 Li 0.18 Ti 0.1 Mn 0.52 O 2 (NFCM-LT) cathode exhibits an excellent rate capability at 3C with 78% capacity retention after 140 cycles in an ASIB. To investigate the material degradation deeply, precycling and postcycling ex situ Raman spectroscopy for structural changes during charge–discharge, scanning electron microscopy for imaging of structure degradation after 1500 cycles, and X-ray photoelectron spectroscopy (XPS) technique for the ratio of Mn 4+ /Mn 3+ in the material are used. The Mn 3+ /Mn 4+ ratio in the material before and after the cycling through ex situ XPS indicates a reduction in Jahn–Teller distortion, which results in extraordinary cyclic stability until 1500 cycles at 10C. The cathode material delivers a specific capacity of 132 mA h g –1 in the first cycle at the current rate of 1C.