Boosting sodium-storage properties of hierarchical Na₃V₂(PO₄)₃@C micro-flower cathodes by tiny Cr doping: The effect of “four ounces moving a thousand pounds”

Na₃V₂(PO₄)₃ (NVP), as a great potential cathode candidate for Na-ion batteries (NIBs), has attracted enormous interest due to its three-dimensional (3D) large open framework for convenient Na⁺ transport, yet its practical application is still limited by its inferior electron conductivity and sluggish Na⁺ diffusion kinetics. Herein, the tiny Cr doped hierarchical NVP micro-flower cathodes (i.e., Na₃V_2−xCr_x(PO₄)₃@C, x ≤ 0.1), which are self-assembled with single-crystal nanoflake subunits in-situ coated with carbon nano-shell, are designed and fabricated via a scalable avenue. The optimized cathode, i.e., Na₃V_1.94Cr_0.06(PO₄)₃@C (NVCP-6), was endowed with more electro-active Na(2) sites and higher electronic/ionic conductivity for efficient sodium storage. Benefiting from these competitive merits, the NVCP-6, when evaluated as a cathode towards NIBs, exhibits an ultrahigh-rate capability of 99.8 mAh·g⁻¹ at 200 C and superior stability of 82.2% over 7300 cycles at 50 C. Furthermore, the NVCP-6 based full NIBs display remarkable electrochemical properties in terms of both high-rate capacities and long-duration cycling properties at different temperatures (−20–50 °C). The contribution, i.e., the design of “four ounces can move a thousand pounds”, here will promote the practical industrial application of NVP towards advanced NIBs.