Constructing hierarchical heterojunction structure for K/Co co-substituted Na₃V₂(PO₄)₃ by integrating carbon quantum...

Na₃V₂(PO₄)₃ (NVP) has been widely adopted as cathode in sodium ion battery devices. Nevertheless, the weak intrinsic conductivity and serious structural collapse limit the further development. Herein, a simultaneous modified strategy of doping K/Co and integrating carbon quantum dots (CQD) is proposed. Substituting K⁺ is beneficial to afford amount of Na⁺ transport within the stabled and expanded lattice. The introduction of Co²⁺ generates beneficial hole carriers to improve conductivity. Furthermore, the bonding of conductive CQD guides to obtain nano-sized NVP grains, reducing the pathway for ionic migration to accelerate the diffusion capability. Importantly, a unique p-n type heterojunction construction is established in the interface between CQD (n-type) and NVP (p-type). This heterojunction structure enhances the mobility of electrons owing to the free pathways, in which the electrons transport in a relatively lower energy level without the scatter and collision of anions dopants. Ultimately, K_0.1Na_2.95V_1.95Co_0.05(PO₄)₃@CQD exhibits with the best energy output level. It's initial capacity under 5C is 109.8 mA h g^-1 and the retention is 87.6% after cycle 400 cycles. Even at 20 and 50C, its output is 93.5 and 82.6 mA h g^-1 for 1st and 66.6 and 52.1 mA h g^-1 for 1000th cycle, respectively. Finally, an asymmetric full cell test confirms its application practically.