Biphasic α/β-Type NaMn0.89Fe0.11O2 as a Cathode for Sodium-Ion Batteries: Structural Insight and High-Performance Relation

Sodium-ion batteries (SIBs) are attracting widespread interest for their potential use in large-scale energy storage systems and electric vehicles. However, cathodes having high energy density often show low power and cycle life due to poor kinetics. The sizable ionic radius of Na+ ion provides a high kinetic barrier to Na+-ion transport. Here, we report the successful synthesis of biphasic alpha/beta-type NaMn0.89Fe0.11O2 that can mitigate these problems. The beta-phase helps maintain long cycle life, and the alpha-phase helps provide good rate capability. The designed material can deliver a discharge capacity of 135.5 mAh g(-1) with a capacity retention of about 84% even after 50 cycles at 0.05C. The diffusion coefficient of the designed material shows 10(-07)-10(-11) cm(2) s(-1) for 40% of the state of charge. The NaMn0.89Fe0.11O2//HC full cell gives an impressive energy density of 200 Wh kg(-1) with an average potential of 2.66 V, when cycled in the range of 1.5-3.2 V. The density functional theory-based analysis shows that the bond length of Fe-O is uneven in beta-phase compared to alpha-phase, thereby leading to a stable biphasic structure. The beta-phase increases cycle life by reducing the effect of Jahn-Teller distortion. The molecular dynamics-based analysis suggests that the alpha-phase helps in getting better kinetics than its "beta" counterpart. Thus, the performance of the cathode can be enhanced by making a composite structure to harness its pros.