Towards stable Na-rich layered transition metal oxides for high energy density sodium-ion batteries

While reversible oxygen redox reactions in transition metal (TM) oxides offer new opportunities to increase the energy density of sodium-ion batteries, performance degradations during the electrochemical cycles due to the metal migration and oxygen loss are the major bottleneck to overcome. Herein, we address this stability issue of the Na-rich layered TM oxide, Na2MO3 (M = 3d, 4d, and 5d TMs and post-TMs), using first-principles calculations for new stable high-energy density electrode materials. We consider thermodynamic stability, oxygen release, decomposition, TM migration, and operating voltage of 38 Na2MO3 compounds, from which eight TMs (Tc, Ru, Rh, W, Ir, PL Mo, and Pd) emerge as significant candidates as the stable Na2MO3 cathode with energy densities of up to 747 W h kg(-1). The key factor that controls the stability of all of these compounds is revealed to be the well-defined phase transformation behavior during electrochemistry with specific cation-vacancy orderings. The new mechanism and several promising Na2MO3 type electrode candidates identified here should be tested experimentally.