Co doping in NASICON-structured Na3.5MnTi(PO4)3 enables superior structural stabilization and rate performance

NASICON-type Na ₃MnTi(PO ₄) ₃ has been regarded as one of the most ideal cathodes due to its high earth content elements and considerable cycling stability. However, poor electrical conductivity, low redox potential of Ti ^3+/4+ and undesirable rate performance remain restrict its practical application. Herein, a series of Co-doping Na _3.5MnTi _1-xCo _x(PO ₄) ₃ (x = 0, 0.05, 0.07 and 0.10) cathode materials are designed via the sol-gel method. By partial substitution with Co, the capacity contribution of the Mn ^2+/3+and Mn ^3+/4+ is increased and the rate performance is significantly enhanced. The Na _3.5MnTi _0.93Co _0.07(PO ₄) ₃ (NMTP-0.07) delivers a high capacity (150.5 mA h g ⁻¹ at 0.1 C), superior rate performance (112.0 mA h g ⁻¹ at 10 C) and excellent cycling performance (94.7 % capacity retention after 100 cycles at 1 C). Benefiting from Co doping, the EIS and GITT affirm the superior sodium ion diffusion ability of NMTP-0.07 and the enhancement of its own conductivity. Moreover, in situ XRD measurement validates the highly reversible structural evolution and both solid-solution reaction and two-phase reaction are involved in the sodiation/desodiation process. This contribution provides a new perspective on the doping studies of NASICON materials.