Partial replacement of K by Rb to improve electrochemical performance of K₃V₂(PO₄)₃ cathode material for potassium-ion batteries

Rb-doped K_3-xRb_xV₂(PO₄)₃/C (x = 0, 0.03, 0.05 and 0.07) composites for high-voltage cathode have been synthesized via a facile sol-gel process. The as-prepared materials have been characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electrochemical tests. SEM and TEM show that the Rb-doped K₃V₂(PO₄)₃/C composites exhibit similar morphology and particle sizes to the undoped one. The electrochemical tests demonstrate that the Rb-doped composites with varied Rb contents deliver better electrochemical performance than the pristine K₃V₂(PO₄)₃/C, where K_2.95Rb_0.05V₂(PO₄)₃/C performs the best. In the voltage range of 2.5–4.6 V (vs. K⁺/K) at 20 mA g⁻¹, K₃V₂(PO₄)₃/C and K_2.95Rb_0.05V₂(PO₄)₃/C display initial discharge capacities of 41.1 and 55.7 mAh g⁻¹, respectively. The corresponding discharge capacities decrease to 32.1 and 52.6 mAh g⁻¹ after 50 cycles, respectively. At a current density as high as 200 mA g⁻¹, K_2.95Rb_0.05V₂(PO₄)₃/C still presents higher initial discharge capacity of 34.5 mAh g⁻¹ with a capacity retention of 95.4% after 100 cycles, while K₃V₂(PO₄)₃/C only shows the initial discharge of 21.1 mAh g⁻¹ with the capacity retention of 85.7%. The improvement in electrochemical performance of Rb-doped K₃V₂(PO₄)₃/C is attributed to the doping of Rb into K sites of K₃V₂(PO₄)₃ and this doping can increase the K⁺ diffusion coefficients in electroactive particles. The results imply that replacement of K⁺ by larger cations is also helpful to improve the electrochemical performance of other K-containing electrode materials for potassium-ion batteries.