Cu²⁺ substitution regulating Na₃V₂(PO₄)₃ with solid SEI membrane for superior electrochemical performance

Na₃V₂(PO₄)₃ (NVP) suffers from poor ionic and electronic conductivity. Herein, a dual-optimized design with Cu²⁺ doping and wrapping in tubular carbon nanotubes (CNTs) is proposed for the first time. This strategy not only modifies the internal electronic structure but also regulates the morphological features of NVP material. Notably, Cu²⁺ occupying V³⁺ sites introduces favorable p-type doping effects. Consequently, newly generated holes can act as charge carriers to improve electronic conductivity. Meanwhile, to conserve the charge balance of the whole system, a series of distinctive Na₃₊V_2-Cu(PO₄)₃ cathode materials are designed. The Na-rich scheme maintains charge integrity, as well as supplying more active Na⁺ to take part in the reversible de-intercalation process. Due to the larger ionic radius of Cu²⁺, Cu²⁺ doping plays a great role as a pillar to support the crystal skeleton and then expand the Na⁺ migration channels, thus significantly elevating the ionic transport rate. Furthermore, moderate CNTs are wrapped around the active grains, functioning together with coated carbon layers to construct a highly conductive framework, enhancing electronic transfer. Meanwhile, the tubular CNTs and porous morphology effectively increase the contact areas between active particles and electrolyte, providing more active sites. Furthermore, in situ EIS measurement demonstrates that a stable SEI membrane covers the cycled Na_3.07V_1.93Cu_0.07(PO₄)₃@CNTs grains to maintain electrode stability and prevent the occurrence of side-effects. Comprehensively, the Na_3.07V_1.93Cu_0.07(PO₄)₃@CNTs sample releases 124.2 mA h g^-1 at 0.1 C. It releases 101.9 and 98.6 mA h g^-1 at 10 and 50 C, suggesting superior rate capability.