Enhancing Kinetics in Sodium Super Ion Conductor Na₃MnTi(PO₄)₃ through Microbe-Assisted and Structural Optimization

Sodium (Na) super ion conductor (NASICON) structure Na₃MnTi(PO₄)₃ (NMTP) is considered a promising cathode for sodium-ion batteries due to its reversible three-electron reaction. However, the inferior electronic conductivity and sluggish reaction kinetics limit its practical applications. Herein, we successfully constructed a three-dimensional cross-linked porous architecture NMTP material (AsN@NMTP/C) by a natural microbe of Aspergillus niger (AsN), and the structure of different NMTP cathodes was optimized by adjusting different transition metal Mn/Ti ratios. Both approaches effectively altered the three-dimensional NMTP structure, not only improving electronic conductivity and controlling Na⁺ diffusion pathways but also enhancing the electrochemical kinetics of the material. The resultant AsN@NMTP/C-650, sintered at 650 °C, exhibits better electrochemical performance with higher reversible three-electron reactions corresponding to the voltage platforms of Ti^4+/3+, Mn^3+/2+, and Mn^4+/3+ around 2.1, 3.6, and 4.1 V (vs Na⁺/Na), respectively. The capacity retention rate is up to 89.3% after 1000 cycles at a 2C rate. Moreover, a series of results confirms that the Na_3.4Mn_1.2Ti_0.8(PO₄)₃ cathode has the most excellent electrochemical performance when the Mn/Ti ratio is 1.2/0.8, with a high capacity of 96.59 mAh g^-1 and 97.1% capacity retention after 500 cycles.