A twofold approach for prolonging the lifespan of cobalt-free Na[Ni0.55Mn0.35Fe0.1]O2 cathode via Bi5+-doping and Bi2O3 coating in sodium ion batteries

A cobalt-free biphasic (P2/O3) layered Na[Ni0.55Mn0.35Fe0.1]O2 (NFM) cathode material has been synthesized and dual surface and structural modifications have been performed. Bi5+ is doped into pure NFM in order to tune the P2/O3 phase, whereas, a thin layer of Bi2O3 is coated on surface of the Bi5+-doped NFM (BNFM) for surface modification. The structure, morphology, and electrochemical performance of prepared samples are analyzed and compared by various characterization techniques. The pristine NFM cathode exhibits the specific discharge capacity of 170 mAh g−1, while Bi-doped cathode exhibits 181 mAh g−1, and Bi2O3 coated cathode renders 180 mAh g−1. It is observed that, pristine NFM cathode suffers rapid capacity degradation and nearly ~80 % capacity loss within first 250 cycles. After 1000 cycles, BNFM shows 47 % capacity retention, while, Bi2O3 coated BNFM (BNMF@Bi2O3) shows 73 % capacity retention of initial capacity. This improvement in the rate capability is obtained due to the effect of Bi-doping and Bi2O3 coating, where, former enlarges interlayer spacing and latter provides the ionic conducting channel as well as protects the particle from the contact of the electrolyte. The combined effect of Bi-doping and Bi2O3 coating facilitates fast diffusion of Na-ions within the transition metal layers resulting in superior rate capability.