Robust NASICON-type iron-based Na4Fe3(PO4)2(P2O7) cathode for high temperature sodium-ion batteries

Iron-based mixed phosphates sodium ion batteries (SIBs) are promising sustainable storage devices due to the low-costing and non-toxicity characters. Whereas, their real application is seriously restricted by the sluggish Na+ diffusion and inferior intrinsic electronic conductivity. Herein, we propose mesoporous sponge-like struc-tural Na4Fe3(PO4)(2)(P2O7) (NFPP) in-situ caged in carbon layer and cross-linked graphene nanosheets (NFPP@C@rGO), toward fast charging and highly stable sodium ion half/full cells in a wide temperature range. This rational strategy gives the cathode superior long-lasting cycling stability, specifically, 86.7 % capacity retention is retained at 20 degrees C for over 30,000 cycles, displaying a capacity loss less than 0.00045 % per cycle. Notably, outstanding thermal stability at 60 degrees C is achieved and no obvious capacity degradation can be observed after cycling for 200 times at 1 degrees C. Furthermore, the impressive electrochemical performances of full cells are determined, ascribed to the improved electronic conductivity and structural stability (e.g. a small volume change of similar to 4.1 % during sodium ion insertion/deinsertion), which is confirmed by in-situ X-ray diffraction (XRD) and X-ray absorption near edge structure (XANES) measurements. All the results above prove that the NFPP@C@rGO is of high potential for advanced energy storage system based on its low costs, long-lasting cyclability and excellent thermal stability.