Na-rich layered Na₂Ti_1-xCr_xO_3-x/2 (x = 0, 0.06): Na-ion battery cathode materials with high capacity and long cycle...

Rechargeable lithium batteries have been well-known and indispensable for portable electronic devices, and have the potential to be used in electric vehicles and smart grids. However, the growing concerns about the availability of lithium resources for large-scale applications have revived interest in sodium ion batteries. Of many obstacles to commercialization of Na-ion batteries, achieving simultaneously a large reversible capacity and good cycling capability of electrode materials remains a major challenge. Here, we report a new cathode material, Na-rich layered oxide Na₂Ti_0.94Cr_0.06O_2.97, that delivers high reversible capacity of 336 mAh g^-1 at current density of 18.9 mA g^-1 along with promising cycling capability of 74% capacity retention over 1000 cycles at current of 378 mA g^-1. The high capacity is associated to the redox reaction of oxygen, which is confirmed here by a combined experimental and theoretical study. The present work therefore shows that materials beyond mainstream layered oxides and polyanion compounds should be considered as candidate high-performance cathodes for Na-ion batteries.