Polypyrrole-coated sodium manganate microspheres cathode for superior performance Sodium-ion batteries

P2-Na_0.67Mn_0.67Ni_0.33O₂ is a promising cathode material for sodium ion batteries (SIBs) due to its low cost, high theoretical capacity, and non-toxicity. However, it still suffers from unsatisfactory cycling stability mainly incurred by the Jahn-Teller effect of Mn³⁺ and electrolyte decomposition on the electrode/electrolyte interface. Herein, the P2-Na_0.67Ni_0.33Mn_0.67O₂@PPy (NNMO@PPy) composite applied as cathode materials for SIBs is obtained by introducing conductive polypyrrole (PPy) as coating layer on the P2-Na_0.67Ni_0.33Mn_0.67O₂ (NNMO) microspheres. Numerous physical characterization methods indicate that the PPy layer was uniformly coated on the surface of NNMO microspheres without change in phase structure and morphology. The PPy coating layer can alleviate Mn dissolution and effectively suppress the side reactions between the electrolyte and electrode during cycling. The optimal NNMO@PPy-9 with 9 wt% PPy delivers a high capacity of 127.4 mAh/g at the current density at 150 mA g^-1, an excellent cyclic stability with high capacity retention of 80.5 % after 300 cycles, and enhanced rate performance (169.3 mAh/g at 15 mA g^-1 while 89.8 mAh/g at 600 mA g^-1). Furthermore, hard carbon (-)//NNMO@PPy-9 (+) full cell delivers a high energy density of 305.1 Wh kg^-1 and superior cycling stability with 88.2 % capacity retention after 150 cycles. In-situ X-ray diffraction experiment and electrochemical characterization verify the highly reversible structure evolution and robust P2-type phase structure of NNMO@PPy-9 for fast and stable Na⁺ diffusion. This effective strategy of using conductive PPy as a coating layer may provide a new insight to modify NNMO surface, improving the cycling stability and rate capability.