Na-site coordination environment regulation of Mn-based phosphate cathodes for sodium-ion batteries with elevated working voltage and energy density

Abstract

Mn-based sodium superionic conductor (NASICON) phosphate cathodes have been considered as new promising candidates for high-energy, low-cost and relatively environmentally friendly sodium-ion batteries (SIBs). Such cathodes, however, suffer from limited Na⁺ mobility owing to rigid coordinated environments of Na⁺ ions at Na(1) sites and low intrinsic electronic conductivity due to the blocked electronic pathways caused by the big size and isolating nature of PO₄³⁻ groups in the NASICON structure, leading to low utilization, poor rate capability and cycling performance. To address the above issues, a facile and efficient strategy to regulate the Na-site coordination environment in Na₄MnV(PO₄)₃ (NMVP) has been reported by introducing K⁺ at Na(1) sites for realizing a flexible Na-site coordinated environment and enhancing Na⁺ diffusion. Combining theoretical calculation and experimental results, it is corroborated that the K⁺ dopant at Na(1) sites can efficiently reduce the Na⁺ diffusion energy barrier and increase structural stability and working voltage. By synergistically utilizing the modulation of the Na-site coordination environment and 3D conductive networks, the optimized Na_3.8K_0.2MnV(PO₄)₃/carbon nanotube hybrids exhibit superior rate capability and cycling performance with 216% capacity improvement at 15C, and 52.5% increase in energy density in contrast to pristine counterparts, and a capacity retention of 81% after 2300 cycles at 10C, revealing their great potential for practical and cost-effective energy storage applications.