Constructing heterostructure by co-doping of Ta5+ and F− with carbon nanotubes to improve kinetic properties of Na3V2(PO4)3

Abstract

The advancement of Na3V2(PO4)3 has been impeded by its subpar kinetic characteristics, resulting in suboptimal electrochemical performance. In this case, a dual modification strategy involving Ta and F co-doping is employed, along with the incorporation of carbon nanotubes (CNTs), is introduced. Notably, the substitution of high-valence Ta5+ for V3+ ions induces an n-type doping effect, thereby increasing the density of free electrons and enhancing electrochemical performance. Moreover, Ta5+ possesses a comparatively larger ionic radius than V3+, which enhances the ease of Na+ transport, leading to an increased rate of ionic diffusion. Furthermore, Ta5+ ions play a crucial role in reinforcing the crystal structure, thereby strengthening the stability of the NVP system. Importantly, during the high-temperature calcination process, the reduction of carbon results in the conversion of Ta5+ to Ta3+, forming a novel conductive TaN phase through its reaction with N2. This TaN phase, known for its high conductivity, intercalates into the NVP bulk as interlayers, forming beneficial grain boundaries. Thanks to this unique TaN//NVP heterojunction, the migration of Na+ ions is notably accelerated along the grain boundaries rather than within the NVP bulk. The introduction of F contributes to reducing the grain size of active particles, consequently shortening the pathways for Na+ migration and improving kinetic properties. Moreover, the presence of carbon layers and the encapsulation of CNTs collaboratively construct a highly conductive cross-linking network, elevating both electronic conductivity and mitigating NVP particle aggregation during high-temperature sintering. Consequently, the modified NVTPF0.07/@CNTs sample exhibits outstanding performance in both half and full cells, underscoring its substantial potential for application in the field of sodium-ion batteries.