Heterojunction of Y-substituted NaV(PO)-NaYO accelerating kinetics with superior performance for full sodium-ion batteries.

The inherent poor ionic and electronic conductivity and relatively low capacity seriously limit the development of NaV(PO) (NVP). Currently, Y substitution is proposed for the first time and corresponding Y-doped NVP samples are successfully synthesized by traditional solid phase method. By replacing V (0.64 Å) of Y (0.9 Å) with large ionic radius, the transport channel of Na is expanded, accelerating the migration rate of Na. Meanwhile, the strong Y-O chemical bond reinforces the crystal structure of NVP and improves its stability. It is worth noting that Y reacts with slightly excess Na during the sintering process to form a new conductive phase NaYO coated on the surface of NVP particles. NaYO possesses excellent semiconductor properties and can form a double conductive construction together with the amorphous carbon layer to improve the kinetics of NVP system. Moreover, the unique heterojunction could be generated between the boundaries of NaV(PO)-NaYO, further elevating the ionic conductivity. According to the investigations of ex-situ XRD and in-situ EIS, NaYO can provide active Na, participating in the electrochemical process to supply extra reversible capacity. Comprehensively, the optimized NVP-Y0.07/C releases a high capacity of 125.2 mAh/g at 0.1 C, far exceeding the theoretical value of NVP (117.6 mAh/g). Even at 60 C, a high specific capacity of 94.2 mAh g can be achieved. When cycled at 250 C, it maintains 85.5 % after 2000 cycles. Besides, the assembled NVP-Y0.07/C//hard carbon full cell can deliver a reversible value of 118.2 mAh/g, suggesting the superior practical application potentials.