Building highly stable and industrial NaVPO4F/C as bipolar electrodes for high-rate symmetric rechargeable sodium-ion full batteries

Abstract

We report the synthesis of carbon coated NaVPO₄F (NaVPO₄F/C) via industrial high-temperature calcination and its application as bipolar electrodes to build symmetric sodium ion full batteries (SIFBs). The reaction mechanism and electrochemical performance of NaVPO₄F/C electrodes as both the anode and cathode have been deeply studied, respectively. This indicates that NaVPO₄F/C electrodes, with negligible structural change and stable valence adjustment, are very applicable for the symmetric system. The electrodes deliver a high reversible capacity of 136 and 134 mA h g⁻¹ with a high ion diffusion coefficient of 3.1 × 10⁻¹¹ cm² s⁻¹ and 2.56 × 10⁻¹¹ cm² s⁻¹ as the anode and cathode, respectively. Moreover, the symmetric SIFBs with NaVPO₄F/C electrodes as both the anode and cathode at the same time exhibit a considerable reversible capacity, good rate performance and long cycle life (capacity retention is 90% after 400 cycles) as designed. This work displays the potential commercial application of the symmetric SIFBs with NaVPO₄F/C bipolar electrodes.