Na⁺-Activation Engineering in the Na₃V₂(PO₄)₃ Cathode with Boosting Kinetics for Fast-Charging Na-Ion Batteries

Abstract

Na superionic conductor-structured phosphates have attracted wide interest due to their high working voltage and fast Na⁺ migration facilitated by the robust 3D open framework. However, they usually suffer from low-rate capability and inferior cycling stability due to the low intrinsic electronic conductivity and limited activated Na⁺ ions. Herein, a doping protocol with Na⁺ in the V³⁺ site is developed to activate extra electrochemical Na⁺ ions and expand the migration path of Na⁺, leading to the improvement of the electronic conductivity and diffusion kinetics. It is also disclosed that the generated stronger Na-O bonds with high ionicity significantly conduce to the enhanced structural stability in the Na⁺-substituted Na_3.05V_1.975Na_0.025(PO₄)₃/C cathode. The obtained composite can deliver an excellent rate capacity of 83.8 mA h g^-1 at 20 C and a moderate cycling persistence of 91.3% over 1500 cycles at 10 C with great fast-charging properties. The reversible structure evolution is confirmed by the <i>ex situ</i> XRD, XPS, and ICP characterization. This work sheds light on awakening electroactive Na⁺ ions and designing phosphates with superior electrochemical stability for practical Na-ion batteries.