Simultaneous defect regulation by p–n type co-substitution in a Na3V2(PO4)3/C cathode for high performance sodium ion batteries†
Abstract
The Na3V2(PO4)3 (NVP) cathode is deemed to be a promising candidate for sodium ion batteries due to its strong structural stability and high theoretical capacity. Nevertheless, its poor intrinsic conductivity restricts further development. To overcome these shortcomings, a dual modification strategy of Mn2+/Ti4+ co-substitution is proposed for the first time. Significantly, Mn doping can efficiently accelerate the transmission speed of electrons by introducing beneficial holes derived from the low valence state of +2, presenting the classical p-type doping modification. Moreover, the presence of Mn2+ with a larger ionic radius can support the crystal to stabilize the Na superionic conductor (NASICON) framework of the NVP system. Ti4+ is introduced for perfect charge compensation. Accordingly, the addition of Ti4+ can generate excess electrons due to the n-type substitution, which contributes to the favorable electronic conductivity. In addition, conductive carbon nanotubes (CNTs) are utilized to construct an efficient network to improve the rate capability of the NVP composite. Meanwhile, CNTs can inhibit particle growth and thus reduce particle size, shortening the transport path of Na+ and promoting the diffusion of Na+. Comprehensively, the optimized Na3V2−xMnxTix(PO4)3/C@CNTs (x = 0.15) deliver high capacities of 70.3 and 68.2 mA h g−1 at 90C and 180C, maintaining 58 and 53.8 mA h g−1 after 1000 cycles with high capacity retention of 82.5% and 78.9%.
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